I would recommend a great book by Alexandre Zvonkine, "Math for little ones", but it is only available in Russian (here); however, two articles by Zvonkine which were published earlier are translated into English, see here. (You might also want to check other materials linked 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.