To even consider anything like this, you need to be a licensed therapeutic riding instructor. Also, the liabilities involved in doing this would require specific types of insurance in case anyone would be hurt. A lot of parents of children with special needs would be very leary about putting their child into the hands of an unliscened trainer. Also remember, not every potential rider will be a child. Believe me, I am a special education teacher. I work with severe/profound students.
I would suggest, if you are really interested in getting involved with therapeutic riding to contact a center in your area and possibly volunteer there. It would be a great experience and you would learn a lot of things. This will be your best bet and in this case everyone can and will be safe.
I would suggest if at all possible to volunteer at a NARHA Premier Accredited Center.
I do not mean to sound harsh or try to upset you. But I want you to gain a good experience and to keep everything safe for anyone involved, including the riders with special needs, the horses, and yourself.
It's not as easy as it seems. Horses used for therapy are highly trained and can take years to develop that type of behavior.
Please don't continue this without first understanding completely.