I love back-matter. Seems that all the juicy tidbits get thrown in there. You can have history, biography, foods, cool places to see, controversy, how you came to write about this (not all in the same book, Hah!) but what fits naturally as an extension. Teachers enjoy these extras and the book becomes more useful for older readers as well.
Definitely keep all your sources but if there's a particularly good resource for children, you might want to include it. Get thee to a library and read some of the back-matter to get a feel of what people include. Verla's books are gold, as are some of the others mentioned.
Sidebars can be tricky. I find these more useful for older readers who can process more than one thing at a time but for the youngest readers, it's good to not break up the narrative thread.
Good luck!