Religion and Church

Here's the problem- she is very involved in her church and goes three times a week to service, she volunteers there as well and she is also a Sunday school teacher so she has to take him with her a lot of those times. I just want him to have a fair shot at him experiencing all religions (or none at all of that what he chooses) and him being in this certain church so much of the time with people talking about their version of God is going to sway him more in that direction which is exactly what I'm trying to avoid. What can I do as a parent to keep him as unbiased as possible? Oh, and I forgot to add, my MIL isn't exactly "young" anymore and just absolutely adores our son and is always telling me how grateful she is to be able to spend time with him and he's her highlight of the day so getting a babysitter instead of her watching him would just break her heart.