I can't imagine how I forgot to mention Leroy "Encyclopaedia" Brown in this list of series that I read as a child. So now he gets a post all to himself. Encyclopaedia is this boy genius who solves his detective father's cases right there at the dinner table. Well, actually he carries out a process of deductive reasoning and leaves you the reader to figure the rest out. After you have racked your brain somewhat you can go to the back of the book to see what answer Mr. Sobol, the series creator, has supplied.
Encyclopaedia often comes into wranglings with Bugs Meany the school bully, but as you might expect, brain generally triumphs over brawn, and by the end of the story, Bugs walks away looking like the total dimwit he is and Encyclopaedia again has earned his name.
As far as I recall, the series generally came in thin paper backs with bold print, maybe six or so stories to a copy and very nice pencil illustration on every two or so pages. I still am looking at some of those drawing right now in my mind's eye. Well now that I am an old (-er) and a meaner critic, the books seem formulaic, but I must admit they were very easy and very fun to read, and they taught virtues that are worth living by. If you have children, or if there is any of the child left in you, you might want to check this little fellow out.