I had heard that the risk is around 5% for a T1 mum and 8% if the dad is T1. I've just googled and found this link:
http://www.diabetes.org/living-with...-diabetes-frequently-asked-questions-faq.html
which suggests 6% chance of a T1 dad passing it on and 4% of a T1 mum who is gives birth under 25 or 1% if the mum is over 25 although the risk doubles if the parent was diagnosed before they were 11.
I have a young son, and this is something I'm worried about, but I keep telling myself that it is still a fairly small risk.