Lettuce and radishes are quick and easy. Children like fast results so that is what I do for my grandchildren.
There are so many books around it is hard to recommend just one. If you have any charity shops near you they are bound to have lots. Typing your query into a search engine or using the search on this site will bring up answers too.As aureliecelerier suggested, runner beans are really quick too. I put them in a big pot with a cane wigwam and there is still time for them.