Even if you know that there is no such thing as original sin and your partner agrees that a baptism is just a tradition but it's important to their family as it is a tradition they feel is necessary. Would you allow it? Would you go to the event? Would you be open with your growing child dispel the nonsense that the child would hear when they are around their christian grandparents? Anyone deal with a partner that's reasonable and doesn't practice the religion of their birth but their family does?
No, but I'm in a different position from most parents. I don't have a partner and technically my daughter doesn't have a dad. I've been divorced for about 25 years and I adopted her a couple of years ago. No husband = no dad. Those with a partner would have a much more difficult time.
Yes, I just go with the Flow... ain't important to me and I will let them know how little I think of the sacrament.
If it was important to my partner, then that would be compelling. Important to extended family, probably not. I imagine everyone's situation is different. If it's really important to them, they will probably do some under the sink baptism when you let them baby-sit. I'm pretty sure my mom did something like that with at least my oldest.