Hi Malibujason,
Generally children do best on a mattress that supports them well (because of their growing and more flexible bones) and has less cushioning than an adult would normally require (because of their less curvy profile).
Good options include an innerspring with cotton or wool padding or a single 6" layer of latex. These are the most natural, healthy, and breathable materials.
Some examples of the general types of mattresses I would be looking at include …
http://www.naturepedic.com/products/mattresses/deluxe_organic_twin_full_queen_mattress_quilted.php
http://sleepez.com/latex-mattress-7000.htm
I would look first for equivalent mattresses at local factory direct manufacturers in the LA area.
Phoenix