The horse isn't being bad, he's just busy and curious...he just happens to be curious with his mouth. What would be great is if the people could find a way to channel that curiousity and use it to their advantage....like teach him to pick up things, like handing them a brush or something. Instead of telling him off for being who he is.
It is fine for a horse to be curious. I find absolutely nothing wrong with this. However, when it comes down to a horse biting, I am sorry but that is not okay, period. What is even worse is that this horse is around children with disabilities. If he were to bite one of these children, much larger issues could arise, that is potential grounds for a lawsuit. These horses need to be solid horses, I mean no issues at all.
I find no problem with a horse learning something new, such as you pointed out picking up things. However, when in comes down to an issue such as this, it needs to be addressed in a way the horse will understand. Horses are herd animals, together they establish dominance. If a horse is going to bite me, they are going to get a quick smack to let them know that their behavior is unacceptable. My horse has bitten me once the entire time I have owned her, she got a quick smack on her nose and has never done in again.
The point that I am trying to get across is that this horse needs to learn that this is not okay before a child gets bitten. Letting him know that the behavior is not tolerated is better for everyone. I am not saying beat the horse, I am simply talking a quick smack to let him know that his behavior is not allowed. Let him be curious in other ways, sniffing, nudging, playing, whatever, just not biting.