Hi Zingara,
I'm actually a Rainbows Guide Leader (in training) and my guide group has decided not too take part in the rainbows sleepover. I think maybe they are a bit too young, and my mentor doesn't think it's worth the stress to attempt it ;)
They will normally stay in the place where the rainbows meet or if it's related to 'the big sleepover' then they'll be with a large amount of girls from other rainbow groups, brownies and guides in a larger guides centre.
From what I've heard though (never been part of one myself, yet!) it's an absolute blast and they absolutely love it.
The brownies often actually go away for a few days at a time too, so I think it's getting the younger ones used to it.
It's entirely up to you as a parent, but I would think if your daughter wants to go and you think she would be okay then I can't see a problem. It might be more of a case of separation anxiety for you?
We are all very nice people at Guides and care about the welfare of the children very much. And these sleepovers would not be done if they hadn't continued to be successful. :)