I used to have the Jack Wolfskin Watchtower. They are up to 20-25 kg I think, so not sure if it is worth it since you could only use it for one season I guess.

Beside of that always bring enough water and for the child a small water bottle that you can refill, stored in an outside pocked of their own small backpack.

Mine enjoyed hiking more if they had a branch in their hand that we found on the way, then they forgot about being tired :D