Rays Mom,
6mp and it's sister drug Imuran, are very good for most Crohn's patients. My Daughter took Imuran for about the last 3-4 yrs.
The one thing that I would highly recommend is, ask your GI about getting a metabolite test. It's a blood test that will tell you what the therapeutic level of 6mp is in her system. It's the only way that they can adjust the dosage to the most effective. It's different for everyone.
If you place alot of your focus on the side effects of these drugs, you will start to get pretty paranoid. I'm not trying to suggest that there's no concerns with side effects, but if you read the side effects to Tylenol, you might never take it again. You just have to keep things in perspective. Most that take 6mp do well on it.