Oh, it's a b***er when people don't handfeed properly right from the start!
With one horse who was way off the scale in terms of acceptable behaviour, I chose to squirt lemon juice into his mouth each time he went to bite.
Sarah
My big horse Flying Fox was a naughty mugger for a little while, who was eager to please and eager to get his reward when he was about 3 or 4 years old, and he had a mouth like a crocodile and a tremendous reach but I found it very successful to use a plastic Jif lemon squibber filled with water. I kept it ready in my pocket. The surprise of water squirted on his muzzle and lips put him right off grabbing or mugging and he became very polite and restrained really quickly and I rapidly was able to discard it. You would have to be very careful not to squirt too near an eye with lemon juice of course.
Without question, to be on the safe side, the children must have it made clear never to feed treats to that horse, or feed treats to any other horse if they are in his eyesight, or they will be at risk of being bitten.