I would recommend a great book by <a href="http://www.labri.fr/perso/zvonkin/">Alexandre Zvonkine</a>, "Math for little ones", but it is only available in Russian (<a href="http://www.mccme.ru/free-books/zvonkine/zvonkine.html">here</a>); however, two articles by Zvonkine which were published earlier are translated into English, see <a href="http://www.de.ufpe.br/~toom/others_articles/engeduc/Zvon-eng.pdf">here</a>. (You might also want to check other materials <a href="http://www.de.ufpe.br/~toom/others_articles/engeduc/index.htm">linked</a> on Andrei Toom's webpage.)

And, as a tiny bit of more general advice, you know about Piaget's works, right? They are highly relevant when trying to teach kids anything at all, I think.