We left DD with my ILs while I had DS, although I went into labour at 6am and he was born at 9am so really, we didn't need to, but I wanted to spend a night with just my new bub in hospital. DD was 16 months at that point. She'd spent a lot of time there, they had a proper bed for her, it was comfortable for her, my MIL didn't mind getting up in the middle of the night to her nor did she mind starting the day at 5am when DD decided it was time to get up

It isn't an easy thing, taking a child overnight, before I reckon about 3yo. They often still have needs overnight... it can be hard for anyone who isn't the parent to respond to them.

We haven't left DS anywhere yet - he's 19mths.

I would just say to MIL "Thank you so much for the offer! That is incredibly generous of you! However, I'm not going to be able to leave him anywhere for a while as he needs to feed overnight and won't take a bottle, sorry!"