I haven't done birthday parties, but I have done some church functions. I can't really help you on what to charge, except that I've seen such services offered for anywhere between $75 and $350 (the higher end is usually riding ponies, or driving horses, and depends on the number of horses involved as well). I'm in California, so the prices might be a bit higher here than elsewhere. I don't have any horses trained to drive yet, so we only do in hand activities when we do things like this at all.
I bring brushes, glitter, and ribbons, and let the kids go to town. You could probably also bring horse safe body paints, hair spray and things like that. Each horse only gets a few kids at a time, and breaks if needed. The amount of kids surrounding any horse is always controlled, and the kids are always warned to be quiet and careful. I also make sure any horse that needs a break is given one. It's usually the girls who stay with the horses longest... they just can't resist braiding the horses manes and tails.
I also bring carrots and treats for the kids to give to the horses. The amount given to each horse is strictly controlled, and because I brought the treats, I know exactly what the horses are getting. Each horse has a handler for safety, and to make sure the horses aren't fed something that could hurt them.
I know I don't have to tell you this, especially since many of your horses are used to being handled by kids, but the horses chosen should be well accustomed to noises, children, and that kind of thing. You never know what kids who aren't used to horses will do. I've had kids run up behind a horse to give him a hug before they went home (as in ran full speed up to Danny's butt and hugged his behind, and then ran away), and I've had kids sit in front of horses, and behind horses (and in one memorable occasion, try to sit underneath Holly).