Somewhere far enough down to be fairly sure of being warm and dry.
But not too far as am a bit worried about towing all that way.
In an area where there's things to do, for a 14yo girl to do. Swimming, either beach or river. Canoeing, zoos. Nice towns to potter round and look at shops, have lunch. Maybe a castle. Maybe some cycling/walking.
Any specific campsites would be good. Don't want rowdy, don't need entertainment. Could do with wifi, possibly a pool.