As a gran, I would love to be asked. But while some grandparentss leap at the chance to watch theur grandchildren; others aren't quite as keen.
You may lack the confidence to ask - or you may have a gut feeling that MIL might not be so keen. You know her; we don't.
Consider giving her an out. Or perhaps asking by email, so it's easier for her to turn you down.
I do hope she jumps at the chance. Good luck!
With luck, she'll also ask her lazy selfish son why he can't look after his own children? For ONE DAY. Yeesh.
Anger management!!! Help needed.