"But Surely That Must Be Uncomfortable?
No, it isn't. Children's skeletons are still largely made of cartilage, and they are far more flexible than adults. Small children naturally sit with their legs bent, and when they are very little this is actually better for the development of their hips and spine. Even older children don't experience any discomfort when they sit like this."
Frome www.rearfacingtoddlers.com/faq.html
I remember looking it up because I thought it couldn't possibly be comfortable, and found a few websites that say the above. I worry about DD getting annoyed that she can't see us, but I'm hoping if I keep her in rear facing she just wont know any different and will be happy!