Hi woofman,
Post #2 here includes links to the better forum threads and posts about mattresses and children and they include links to some good quality/value options and sources as well. 6" of latex can certainly make a good choice for children … particularly if it includes a wool quilted cover which can help with temperature regulation and breathability. This topic may be worth reading as well about choosing a medium or a soft top layer.
If you decide to order all the components separately then the component post here includes some good sources for mattress covers.
Phoenix