Stella Management has a great summary California child labour laws and work hours for film sets here:
http://stellapacificmanagement.com/?page_id=8
Though your production may not be based in California, for early planning purposes anywhere, this page gives you an excellent overview of what hours you can expect are reasonable for child performers. Of course as you near production, ensure that you abide by the local laws.
Cheers & a good shoot to you,
Deb