Do you specifically want one that goes on the back? I didn't like the idea of not being able to see them, plus it does funny things to your centre of balance, so we got a
weeride - we love it, very stable and seems much safer (check out the multimedia info on their website, there's an embedded YouTube vid of a Sunrise story about different types of bike seats). All bike seats are recommended for 12 months and older although the main thing is that they can hold their head up fine with the helmet on

I was surprised at how expensive helmets are but if you have the right size it stays on well.