I think you need at least a few 'deadheads' in the mix. While I think it's a good idea to have kids actually thinking about the horse as a living, feeling thing, some special needs are unable to really think in those terms or control themselves (kids with seizures especially) and will cause even a well-trained but still reactive horse to be unnerved, confused, or annoyed. Bombproof (and I use that term lightly; every horse has their kryptonite) horses are not necessarily unresponsive to the handler and if they are, they can be retrained.
But that's beside the point. I would talk to the woman who wants you to train the therapy horses about what exactly each will be doing, and then desensitize them accordingly. Will there be sidewalkers walking with the horse for a lot of lessons? Get them used to people being right next to them, touching them, bumping into them, etc, at a walk and trot. Get them used to wearing all different tack - handles, therapy saddles, no handles, endurance saddles, whatever they'll be wearing during therapy sessions, and ride them in all of them. Teach them the slow, rhythmic 'therapy jog' pace. Make sure the horses are polite and responsive to their handlers. Have the horses work with a lot of different people. Have mock therapy sessions where the rider does a variety of things that special needs people might do - yell, wiggle, rock back and forth, flop to the side, sing loudly, kick or hit the horse (of course, not hard). Do these inside and outside with a variety of handlers, sidewalkers, tack. You can also crinkle a water bottle and drop things while on their back, etc.
Basically, throw everything at them that you can think of and see how they react.
That's all the stuff I can think of to do with them right now - lots of desensitizing. It sounds like the lady you're going to work for is looking for a 'golden' horse (sensitive to rider but unreactive at the same time), and those are very hard to come by and will take tons of training. If you're gentling a mustang right now I'm sure you know what you're doing as far as training a horse in the general sense. I would just ask the lady what the horses will need to know before they can become therapy horses.
Best of luck!