When I was a teenager, I was diagnosed with a 90 degree Scoliosis curve...add into the equation Lordosis and Kyphosis and the 'osis' family lived in my body. I had Harrington rodding done in 1976, yep, back has been fused for over 30 years, this year, and I have suffered from chronic pain ever since. The upside of the pain is that I would have not lived too long, if I had not had the surgery..so I do have my wonderful parents to thank for chosing that decision for me, I was 18 when I had the surgery.
I do know that a long time ago, physical therapy was not as proactive as it is now, and I was told I could not do a lot of things, and I believed them...I now walk a lot, lift a lot, have good muscle tone in my arms, but not a lot of upper body strength...in fact, I am a size 6 on the top, and a size 8 below. I essentially believed the 'experts', until I got on a horse in my 40's, and my spine did not crack...lol. Anyway, I have had a lot of other health issues, that are mostly bone related, long story, not many broken ones, but due to whatever God gave me to deal with. I have always been one that if I was told 'No', I would find a way to do it.
I think that if your child wants to ride, go for it. I was not allowed around horses until I was much older, as everyone was always concerned I would ride, and get hurt. I have gotten hurt, but my back wasn't! That is one of the reasons I chose to ride the gaited horses, due to the no trot, as I don't sit that one comfortably at all, and my knees are not tough enough to post.