Originally Posted by
Mike Mills
My experience with "kids" is that they have their own personal preferences regarding seat height and are usually uncomfortable with seat heights set to the "proper" height. I'd advise you to bring some tools and slowly raise the seat height over time (once per day?). Let them start where they are comfortable but work up to the proper height over time.
They also have an unnerving habit of suddenly growing! (Based on experience of two sons and continually having to replace outgrown school trousers on a far too regular basis). As far as bike sizing goes, the 'book in the groin against the wall' method would seem to be simple and adequate. Even if they grow a half inch in leg length in the remaining weeks, that can be accommodated by seat post adjustment.