Bicycle Tutor

I bought a second-hand first bike for my grandson, and the saddle is a one-piece element with the seatpost. The saddle needs replacing but unfortunately the post is very narrow - 20mm in diameter. Does anyone know where to to buy either the all-in-one saddle/20mm post, or a 20mm post to which I can fit a saddle?
Thanks in advance for your help/advice.
Martyn

If you have no luck with sorting your saddle.
How about one of these:-
http://www.wallbike.com/accessories/clamp_ideale.html
Cut the top off your existing post and using this style of clamp you can fit a "normal" saddle.
The only thing is the clamp is designed for 7/8" posts (22mm) but may clamp to your 20mm post.
Some saddles come with clamps but are not always used, so ask at your local bike shop.