Pleased to see no-one's mentioned locking the door in some way - just to reinforce for anyone that may come across this question, that's definitely what not to do, since in an emergency you want to be able to get out as quickly as you can.
My first instinct is to ask if it's even a risk that's worth accounting for? Unless you're talking about particularly adventurous or mischievous children many will be too scared to venture out into a dark, unknown, unlit area without knowing their way back.
If you decide it is then there's a number of things you can attempt to do, though realistically the only thing that's going to be dead set in stopping them is sleeping in shifts. First, make sure the adults sleep closest to the door and the children closest to the back - this way they'll struggle to get out unnoticed if they have to climb across 2 or more fully grown adults without waking them. You could also use a GPS / radio signal tracker, but this relies the child keeping it on them which they presumably won't do if they think they're not allowed out.
Another point in relation to this though is to lay down the ground rules, and lay down the consequences. If they know the possibility of them getting caught is there, and this possibility would result in wrecking the trip somehow for them all, they'll generally be a lot more cautious about attempting to sneak out!