I would let him grow and mature at least until next year before asking much of him. You can do a lot of work on the ground in the meantime, however.
As for the small children...yikes! If it's getting some weight on his back you want I'd use a western saddle that fits and if he is okay with that then add some weight with saddle bags attached where a rider's weight would be. If he pitches a fit then at least it won't be the small children that go flying.
If you have never started a green horse the next few months will be time for you to seek out help and/or increase you knowledge. Not meaning to be insulting if you are experienced. It's just that your post didn't give any background.
And welcome to the forum!
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger