I agree ignore the other parents -- you know what you are doing and what's best for your daughter.
I do agree with others about needing a backrest for her though. I would not ride on our Spyder as a passenger without one.
I respect what your doing for your daughter and would be willing to make a donation towards the purchase of one

I think it's a great reward for a special girl. Having a disabilty myself and knowing the work it takes to do rehab to get better I always enjoy getting out to take a ride as a reward for myself

I'm sure others here would love to support as well !
Another idea for a removable backrest and storage is here :
http://www.spydertalk.com/forums/topic/13269-13500-tour-pack/