Hi there, we wanted to hire bikes in Lijiang...we have a 10 and 12 yr old, just wondering if anyone knows of a particular place we could hire kids' bikes, and maybe even a helmet for them :-)

The places that rent bikes will have bikes for small adults. If your child is Caucasian and normal size for that age, they should have no problems finding a bike.
You could also try contacting these people www.bikelijiang.com/