Amazon.co.uk sell books for juniors, and I've seen junior sudoku books in Waterstones and Dobbies the garden centre(!). Not being a junior and not having a junior, I don't have occasion to pick them up and look at them so don't know quite what age range the junior category accomodates (or the quality of the puzzles), but I'm sure that Amazon will be able to give you an idea. I certainly know that Carol Vorderman does a junior book and her other books contain puzzles by Nikoli, which are of a very high standard.
Also, the program that is sold on this website caters for juniors insofaras it creates 4x4 puzzles for the junior members, if that's any help. You get a free 28 day trial to see if it suits you.