So if you go to a book store in Japan there's hundreds of books for kids to practice their kanji in. Does anyone know what the best series is and what makes it different than the rest? I have a little brother and sister in 2nd grade and I'd like to work with them to get them ahead of the curve.

I've tried a few and liked the kumon series best, but there are excellent online resources as well like http://www.jakka.jp.

For example:
http://www.jakka.jp/mondai/gakukanji/6/60826.pdf
http://www.jakka.jp/mondai/gra/6-1r%201.pdf