We've done tons of sleepovers, though none outside the house.
I definitely put a time frame on it too. Most of our sleepovers are from mid afternoon till mid morning-ish. I tell parents to come pick up the kids at 11am the next morning (or earlier if you think you'd go crazy before that). I find the kids pretty much take care of themselves during the actual sleepover. Especially at 9 yrs old. I'll organise 1 or 2 activities for the afternoon (crafty things for girls, or nail painting etc) IF they want to do them. Sorry, no idea on boys activities. I give them something super easy like pizza and chips for dinner, then they watch a kids DVD before bed. Once they're in bed, they'll want to talk and laugh and be silly. I give them an hour or two to do that, afterall, that's half the fun of sleepovers, then they have to go to sleep. In my house that would be about 10 -10.30pm ish at the latest. They'll be awake at the crack of dawn anyway.
In the morning they either watch another movie or TV till breakfast time (this helps keep the house semi quiet if it's still early), then just let them play till parents pick them up.
The time will actually go by really quickly.
As for sleeping in the yard, it would depend on how safe the area is, and where the closest parent would be. I would think a parent would have to be sleeping somewhere nearby (though not necessarily in the same tent) for me to be happy sending my kid.
Oh, and I always tell the kids/parents when they arrive, that if they're not comfortable sleeping over, they can go home whenever they're ready, then return in the morning for more fun. No pressure. I find that 99.9% of the time they end up staying.