Hi ImAbsorbed,
I don’t have any specific recommendations because there are dozens of mattresses that would be suitable for a child that use different combinations of materials and that are in different budget ranges.
Post #2 here and the topics it links to have more information about mattresses and children and materials that are “safe enough” for children including a link to some general guidelines for children in post #2 here. It also includes a number of links to the better forum posts and topics about mattress and children as well which have more information about many good options for children which would also be worth considering.
Phoenix