I will start by saying that this is absolutely not a MIL-bashing thread, mine is lovely and I think the world of her!
I have left my 20 month old with her a few times now, mainly because she has been very keen to and it's been a help when I've had to go to appointments etc. I really appreciate the help. However, a couple of things have worried me a bit and it's made me nervous about leaving her. It's mainly that she doesn't properly toddler-proof her house before visits - eg. I once had to stop DD from picking up an ant poison trap. Yesterday I got back and DD was eating a bowl of whole grapes! Are these things that people really should know, or have times just changed since MIL had young children? I have obviously gently mentioned things as I've noticed them, but it worries me that one day something will happen. Am I just being a bit precious? I'm not sure.