What do you mean by therapeutic riding? Do the children have physical disabilities, cognitive disorders, ect?
I think it would depend on the child and their motivation. If all they want to do is ride then I would think the motivation for groundwork would be less. Maybe you could work a little groundwork into each lesson prior to riding. They would know the riding part is coming and be more motivated.
Depending upon the abilities and ages of the children, and personalities of the horses, some types of groundwork may be very difficult for them to have success with, and cause frustration.
All that said I am a fan of groundwork. When I have my kids do groundwork I make it as interesting as possible setting up obstacles, playing Simon says, red light green light, and make it fun for them.