dear kay,
Children do not have backaches very often.
Children with hyperlaxity can get pains and aches when they " overstretch their joints". I suggest you do a good observation with the necessary tests and if you do not trust the outcome, send your patient to a colleague or specialist to evaluate the source of the backache.
It could be a posture problem, it could also been a structural problem and you would want to know. Making goals for efficient therapy is only possible after a extensive assessment.
The therapy will have to be fun to do and be repeated at home a lot. In the case of a nine year old, that means getting the parents involved for 100% as well.
Hope you find the source of the problems and good luck to you both.
esther