2.5 yr old sleep over

[teresac]

what age do people think it's appropriate for children to stay overnight by themselves at their grand-parents(-in-law)?

I would wait until your DC is old enough to have an opinion. At a certain age (variable according to DC) she will have worked out that grandma's house operates very differently. She might be the sort of DC who can adapt easily and just takes -2 houses, 2 sets of rules-in her stride or she might be the sort who doesn't feel secure in that situation.
It is delicate-I think I would stick to saying that you are waiting until DC is older and happy to make her own choices.

excellent advice. thanks