Welcome to HW,
YOu've come to the right place to get some very good advice, from many experienced individuals.
I totally understand your concern regarding the use of 6mp in such a young child. I struggled with the decision when my Daughter was 15, so I can imagine how much more difficult it is at 9.
When researching some of the drugs that are being used to treat Crohn's/UC, it can seem pretty scary. If you took the time to research Tylenol, it would seem pretty scary too. The current crop of immunosuppressants, do carry a certain amount of risk, but many of the severe side effects that you've been told about are mainly at the extremely high doses that are used with transplant patients. We really don't fall into that category.
My Daughter has been on Imuran, which is the sister drug to 6mp, for about 5yrs. now. The only side effect she incurred was hair loss. Unfortunately for her, the Imuran didn't seem to keep her in remission over the last year, and we've now had to progress to Remicade.
While these drugs are certainly a concern, the damage that the disease can do if left undertreated, can be a much bigger concern. In hindsight, I wish my Daughter had of gone straight to Remicade, which is referred to as the top down approach.
The one thing you need to know about 6mp is, that it has to be at a peak therapeutic level. She will need a metabolite test to determine this after she's been on it for awhile. This is how they adjust the dosage to make sure it's working. You'll also have to be patient, as it can take up to 6 months to start working.
Good Luck with everything. Hopefully you'll get things under control soon. I'm sure you're not unlike alot of us, and can be consumed with worry at times. Your Daughter may present as a very mild case, and enjoy a very normal life was properly treated.