As I understand it, the risk of passing on CD or similar is the same as if you have any auto-immune disease - statistically, you're more likely to have a child with an auto-immune disease, but I don't think I've ever seen anything that quantifies that? And it doesn't mean to say that your child will have CD necessarily - he/she could miss it completely, or it could show as asthma, eczema, etc.
I was considering a second child when I was diagnosed, and I was told that there's no way of telling how my CD would be when pregnant - could be better, could be worse, could be just the same.
What I wasn't told was that some drugs (like Imuran) are contraindicated when TTC or pregnant. I fell pregnant nearly 2 years ago, and my doctor's opinion was that I should seriously consider a termination due to the foetal damage that Imuran would have most likely already caused. He also said that in most cases, the pregnancy is not viable. I researched it, and got conflicting information at the time re: foetal damage. As it turned out, it wasn't a viable pregnancy, but I made the decision then not to take the risk of putting myself and dh in the position of having to possibly make a very difficult decision if it happened again.