Lots to be said on this topic, that’s for sure. I do think that it would be appropriate to take your daughter to Saturday night mass the night before she’ll be going to a protestant service. I also think that if you were an atheist, Muslim, Jew, Hindu, etc that Sunday School would be questionable, but as a Christian, Catholic or not I don’t see a problem. If it’s anything like the Sunday School I went to and later taught its just a fun, back to basics time where kids are taught bible stories in a way that is more on their level than a sermon is. I don’t know why you think this would be against Catholic teaching, they aren’t going to discuss Mary, the saints, the Eucharist, etc. Most protestants are too busy being protestants to waste time knocking down Catholicism to children, most of whom wouldn’t even be able to say exactly what a catholic is. As kids get older Sunday School usually involves learning how to incorporate Christianity into your daily life at school and with friends. Things like sharing, being a friend to someone who is lonely or picked on, honesty, and standing up for what is right are covered. It really is just an hour of fun, basic religion. Theology isn’t really covered. As a note: I’m an atheist and I can rattle off the 12 disciples faster than any Christian I know, because of a song I learned in Sunday school 20 years ago!
I hope I’ve at least helped to put your mind at ease because whatever you decide to do you don’t want to do out of a knee-jerk fearful reaction. On one hand a Protestant service probably won’t confuse your kids if you explain in an age appropriate way that protestants love and worship Jesus, they just don’t have the full understanding of him. It’s good to pray and celebrate with them, but it’s more important to always go to mass and that if they ever have any questions they need to come to you. Then instruct your parents that any questions they get asked they need to defer to you, and if it is something specific about their church you can answer it together.
Yes, you do have the right to put your foot down, but if your parents are good grandparents to your children and help you out you may want to tread carefully and choose your battles. My husband and I will be raising our children to not believe in god. But we know that realistically there will be the occasion where they need to spend the night on Saturday with their catholic grandparents. We want to keep this from happening as much as possible, but life happens. It will just be a matter of timely and age appropriate “damage control” after the fact, and it isn’t even like we are all Christians.
But if you decide that your child should never be exposed to Protestantism then talk with your parents and see if a compromise can be reached. See if you can make it easier for them to see the kids on other nights, or if you can pick the kids up before church on Sunday, or if you can have a friend or other relative do it for you. Just try to keep in mind that it is generally very beneficial for kids to be close to their grandparents, so your goal should be to foster that relationship while maintaining your values.