These are the front fitting bike seats aren't they? I have a different version...they are BRILLIANT, I definitely recommend them. Much better than the rear fitting ones because you can see what they're up to and chat with them as you go along...and off course it keeps the back of your bike free for shopping. The only potential problem is that fitting one could interfere with the cables on your bike so check where it would lie first.

My son was in his till he was 4 1/2 and then progressed onto a simpler saddle that fits onto the cross bar but I will definitely be getting it out again when no. 5 arrives later this year.

I got his helmet from argos but anywhere you can buy the seat should sell them in the right size. HTH's