I think its allabout the individual child. My oldest just spent thenight away from us AND away from his own homefor the first time less than two weeks ago. He's 7. It was his choice to spend the night with his grandparents while my DH and I went away. He had a good itme. He's known his grandparents since he left the womb and we live on the same land as them. If he'd wnated to come home, he could have, my mom was here with my toddler. He's stayed without us before, but, he was at home with my mom. My toddler stayed here with my mom for the two nights we were gone and didn't notice we were gone. He was in his own home, slept in his own bed, played with his own toys, and was with his Abuela, who he's completely attached to.
WHen a child is old enough to ask to stay somewhere else, they ar eprobably old enough to do it, with some explanation of what that means to them. If I were uncomfortable sending my little one off somewhere, by all means, they probably woudl'tbe going. Of course, if that little one was 15, bigger than me, and very persuasive, maybe I'd sing a differnt tune

Seriously though, if it's bugging you, stand firm anddon't let her go. Maybe let her go spend an entire day with them, you know, morning til night, see how that goes. Can they get her down for a nap if she needs it? HOw do they handle her when she's upset because you're not around? Things like that. And, it's really hard to let them do things like that. We didn't let our oldest go of for the day with his grandparents until he was 3 and don't plan on letting our toddler until he's 3 either. By then he was big enough to ask questions, talk,all that stuff and I think that's very important too. Good luck in whatever you decide!
Namaste, Tara