The ideal thing, I guess, would be a French playgroup or something - in my city there is a French Institute that runs play sessions "in" French, I believe, mostly for French families living here but also welcoming children wanting to learn French. If that's impossible, you could try one of the DVD-based French learning packs, e.g.
Salut Serge. We have this one and I quite like it, but I couldn't really say DS learned much French before he started learning it at school; otoh, he never especially wanted to, so ymmv.
