The one biggest obstacle with kids I have found happens to be that they have to rely on a parent/adult to get them or their equipment to the field/flying site. Then we have had some parents think we are a baby sitting service which we make clear we are not. Due to liability and the possibility of injury and discipline the parent/adult must remain with the child.
I have just this past year arranged a free membership for two brothers (one confined to a wheelchair) in both the club and the national organization. I made available a PT 40 converted to electric as their trainer. I phoned every week trying to get together Wednesday or Sunday but the mother always has an excuse or something else that needed doing. We even fly indoor in the winter on Sunday and she could never make it.
The longest lasting successes have been when I can convince the parent/adult to join and have the child join as a family member (ours join for free under 18 yrs old) and teach both to fly.