neither of my kids are christened and my wife is happy with that too thank god.

I tell my seven year old that god is made up but santa is real. presents are proof and, until there's evidence to the contrary santa is a good theory.
My daughter loves to sing and when I catch her singing a song she's picked up at school, thanking jesus for her house or the food on her table I tell her she wants to be praising me as it's me providing those things. now, go and get daddy a beer from the fridge
Let me run this one by you. Your wife's mum is very religious, and is aware of the above arrangement (non-christening). She and grandad offer to look after the kids one Saturday. Then, they take the kids away in secret to a pre-arranged church and have them baptized without you knowing about it. How would you react?
This happened to someone close to me. I would be pretty enraged, to say the least.
I would go absolutely feather spitting mental