Both my daughters did ballet for a number of years, and I think that it helped with their general fitness and muscle tone. However, when the time came for them to start work on their points, I refused and they changed over to modern ballet, which they enjoyed judt as much. For my daughters we felt that being en pointe would just put too much strain on their knees (described by their teacher as banana knees) and their ankles. Their teacher was happy with our decision and made no attempt to dissuade me. I do think that it is impossible to say that ballet is good or bad for peeps with hypermobility as like everything else, it depends on the individual, their condition, the level of ballet, the teacher etc.
I do think that for many young children it can be a good way of strengthening muscle and improving or maintaing muscle tone, but it does require monitoring, and may need reconsidering if it causes injuries and/or pain for the child.
Sorry i can't help more
Diagnosed HEDS December 1st 2005. DD1 (20) HEDS and scoliosis (now corrected by surgery), diagnosed June 2006. DD2 (18) mild HMS. Son (11) some hypermobile joints, poor muscle strength and seems to be developing scoliosis as well, woopee!