I think it is wrong that she sprung it on you at the last minute when she knew earlier.. That would annoy me to, especially when she knows the way you feel..

FWIW, I would leave those age kids and that many with my inlaws, even MIL on her own, especially when the older 2 are well behaved and help keep the younger ones entertained if she had to be out of the room, changing a nappy or the like..

But since my parents separated, no I wouldn't leave them unless it was an emergency.. Dad just wouldn't be comfy, and Mum is a bit scatterbrained at moment, so not worth the risk..