If you'd never done one before, but were planning a sleepover

[earlgrey]

in the garden for four 8 year olds, what things would it be useful to know?

Useful to know that you will not get much sleep!
Make sure they have a couple of torchs for when it gets dark.
Put the tent fairly near the house and leave the back door open so they can come back in.
Is it girls or boys?