I think I'm missing something here. You say a smaller horse would be one you would be riding, but the disabled children would be feeling more comforatable on? Are you saying the children would be riding on the same horse with you? I'm not sure that is a good idea, if the case.
Started a theraputic riding school, takes a great deal of planning. It is important that the center be fully accessible to physically handicapped adults and children. The facility should have handicapped accessible barns, bathrooms, and parking areas.
It is very important to find suitable horses that are as bombproof as possible. The horses will have to deal with a number of unexpected movements and sounds from the riders and assistance staff, so a steady disposition is critical. Horses should also have smooth gaits and be well trained for beginner level riders.
The riding arena should be completely fenced for safety, in case a horse should bolt unexpectedly. A covered or indoor arena is good to have, when weather conditions are not idea. Mounting blocks, ramps, or automated lifts might be needed for assisting riders as they climb up into the saddle.
It is important to obtain an insurance policy that covers liability and other potential needs. The coverage will protect you in the event that someone is injured by an animal or is in any other way injured on the premises.
You should also check into the possibility of licenses being required by your city and state. You may need a business license and file additional forms to operate a therapeutic riding facility.
You will have to have at least one, Certified Instructor on hand, at any time a disabled child is in your care.The most prominent certification group for therapeutic riding instructors, is the Professional Association of Therapeutic Horsemanship International (PATH) , formerly known as North American Riding for the Handicapped Association (NARHA). PATH instructors are certified in three levels: Registered, Advanced, and Master. There are 3,500 PATH riding instructors and 30,000 volunteers that currently serve over 42,000 students worldwide.
The majority of your workd with disabled children, and according to their level of disabilities, will be done in the arena and not out on trails. Out on trails, you will have very little control, should something go wrong. Safety of the children will always have to be your number one concern.