My daughter has been on 6-MP for almost 2 years and is doing very well.
The best time to start 6-MP is while on Prednisone and then to slowly wean off the Pred as it can take about 3 mo for the 6-MP to kick in.
We have had no issues- knock on wood.
My daughter also can not swallow pills so I crush it for her and put it in pudding or whipped cream etc. The pill is not very big but if you can't swallow it it can be crushed.
Our GI won't let me daughter go longer than 3 mo without checking her labs and seeing him.
We also use the probiotic VSL#3 and I really do think this helps quite a bit.
If ever she seems to be a bit off I never hesitate to use hydrocortisone enemas for a day or 2 to get us going the other direction.
My daughter is able to be a normal kid and i am just so grateful everyday that she is doing so well.
Best of luck that 6-MP will work for you too!