Wowo was good fun last time I was there - very much set up for families, allows fire, rope swings in the trees and next door to steam trains - all of which may be of interest to 5 year old boys (and other OTP!). Their web site isn't talking to me, so whether they'll do a one night booking I don't know (and from what I've seen not many places do one night on a summer weekend.)
Also registering interest in the thread as I'm planning the odd night away with Miss Dans Younger and Elder this summer. I might even have a go at cycling / hiking to a site with them, if I can pick somewhere in really very easy reach. (Neither is a habitual long distance rider and I'll end up with most of the stuff.)