Do nothing
You’re not the parent
Just be fun grandpa and support everything they do
They will come to you on their own when it’s right for them

We all need to walk our own path

Don't worry - I took those classes too! Yet here I am.

Frankly CCD classes made me question things way more. They weren't taught by terribly bright people - and the things taught didn't make a lot of sense. Perhaps they've upped their game since then - but I seriously doubt it.

And yes science helps. Perhaps a magazine subscription - depending on their age group? Smithsonian - National Geographic... or something aimed at kids if they're younger.

We always had a chemistry set in the house.

I also recommend getting them a copy of "Hitchhiker's Guide to the Galaxy" as "one of the best funny books you've ever read".
It took me years to realize Douglas Adams was an Athiest - but there's some stuff/ in there that will get a young mind thinking. Plus frankly it's FUN! (If you haven't - read it first - so they can talk about it with you).

Just ask "innocent" questions, As If trying to understand what they are being taught...the glaring weirdness should become quite apparent quite quickly. Then you can ask them how they feel about it all....

Just encourage them to be curious about everything and have enquiring minds. We have had our turn at raising children, as grandparents we must let our own children raise theirs as they see fit, unless they are neglecting or abusing them, we must stay out of it.

It's not your place to counter act anything - these aren't your kids and while you may see this as some form of abuse - it's not the same as real physical or sexual abuse. Mind your tongue and just enjoy your grandkids when you have the opportunity to. Otherwise, if you interfere the parents could limit visits and you will be the poorer for it. I'm a grandfather also - I may not agree with everything my children do in terms of raising their kids but until it constitutes real abuse - I'm keeping my thoughts to myself because it's not about me.

That's what I've been doing the past few years.
Science kits, do-it-yourself robots, cookbooks to make "gross" stuff - science
experiments with food.
Also, I take them to Marvel movies. Then, I can explain that "gods" are
like superheroes. I also tell them about Norse mythology, and we watch
shows about ancient Egypt.
I constantly tell them that all gods are myths. So far, they haven't asked
about "the god", but if they do, I will.
Santa and Tooth Fairy are still coming around, and I will not be the one
to spoil that little bit of childhood for them. Not to mention, it would really
piss off their parents.
We do what we can.
Good luck.

I've been that little kid raised in Christianity. It wasn't catholicism, but it was a strict form of Christianity. What swayed me most was seeing atheists be good people. I was taught no one could love apart from Jesus. I saw Atheists love. I was taught all good deeds were "like filthy rags." I saw atheists doing good deeds and not caring about the reward, while Christians seemed to only care about God, not the people they were helping. I saw atheists react better than Christians, love the earth more than Christians, love people more than Christians. That's what started my questions. If God were good, why are some of the people he calls "evil" actually better than he is?

Then came science. With that little foundation of questioning and not being afraid of atheism, I could look at science differently. I could learn that maybe there are explanations out that that explain human behavior and natural observations better than this little book I was told never to question and had to squeeze into any scientific teaches I happened to be exposed to.

Just be an awesome grandparent. That is enough to make little minds open.

Give them as presents subscriptions to National Geographic for kids, and as they age switch to the adult versions of National Geographic and also the Smithsonian.

I was raised a Roman Catholic and spent my entire childhood believing every word of it until I reached an age in my teens when I started to question what I'd been taught. The rest is obvious.

Just because they're raised with it doesn't mean they won't be able to see through it eventually.

I would suggest simply engaging in curious questions. “I wonder, what do you think about that? Does that resonate with you?”’

It is science that saved me from religion.

Question everything. Test everything. Go with facts and evidence, not blind beliefs. Studying the Bible scientifically, I discovered that the prophets had no gift of prophecy, that there are numerous contradictions in the Bible, that many stories are very unscientific, such as the 6-day creation and Noah's Ark, etc.

Peak his curiosity with things like a rock or bug collection. Books about dinosaurs, engines, how things work...you can be a great influence on his life without stepping on toes...my nieces and nephews loved their grandpas for being honest, fun, and giving them views that they will look at when they get older.

Try and foster a sense of questioning everything. My late partner's mother fostered that in her and she always thought religion silly. Unfortunately, when they immigrated to this country she simply was more concerned with an education and learning the culture and didn't promote critical thinking with her kids. She later saw where that led (2 became S. Baptists) and regretted it. However, be careful as her daughter-in-law (a very strict S. Baptist) saw her as dangerous and from day wanted to have nothing to do with her. In the end my partner was basically pushed out of the life of her grand kids.

Set a good example of being a good person yourself, and encourage aways the asking of questions. Some here have said that it is not your place to preach any skeptical position, perhaps not, but you can and everyone always should push and propagate as hard as possible questioning. Some have said encourage science, but the real value of science, is in its philosophy of questioning and that is more important than any practical experimental outcomes.

Introduce them to people of other faiths, and if they get on, ask them how they feel knowing that people they like will not get the same salvation they will. And you never know if you get lucky, one day they may ask you how you can be moral without god, then you can answer with all your heart.

And fossils. Buy them fossils.

Be supporting of them even if they become Catholics. Could be worse, they could become drug dealers or people traffickers when they are older. With a firm foundation Catholics can become great people, the same as everyone else, even agnostics and atheists!

Support them not subvert them.

Well, many of the most famous scientists were catholic. I love practicing science but the religious will always remind you of this. I think a better strategy is to instill healthy skepticism. When they ask questions, don't just give them answers, walk them through the process of figuring out the answer. Turn the "why" game around on them. I think skepticism and curiosity is the antidote to religion; it was for me at least.

Books -- good books, and lots of them: Mark Twain, Kurt Vonnegut, Catch-22, science fiction, science, poetry, ...

How old are they? I homeschool my 9yo so I’ve got an abundance of books grounded in reality, as it were. There is one leisure read called “How to think like a scientist”. A book about evolution, without a menacing title is “Little changes”.

I had my kids went through this even though I had already decided and started talking to them about other beliefs as I grew up catholic. I made them finish the class because I told them if they ever decided it was for them they wouldn’t have to go through the class as an adult. Lol. Catholicism has many rules and hoops to jump through.

My nieces/nephews were Xtian homeschooled. After my first trip to Egypt, I got invited as a ‘guest’ speaker to talk about it. Lol, I didn’t get invited back, but it opened a discussion that continues with my eldest nephew, who is also atheist. The others range from believers to meh.

Tell them to abstain from sex until birth control is permitted. Also, SELF-DEFENSE lessons, maybe martial arts?

Little, plastic dinosaurs should get them thinking. And model rocket kits are a real blast!

If religion can be sneaky so can science.

Kids don;t listen to what you say. But kids watch everything you do, so set a good heathen example for them. Don't criticize mom and dad, but make it clear to them that you rely on reason and evidence, that praying is just talking to yourself so don't ask for something you can't do yourself, and live your life like that. Good luck, partner.