Hi TossingSquirrel,
Post #2 here and the topics it links to have more information about mattresses and children and “safe” materials 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 may be worth considering.
For younger children any mattress that uses materials that are “durable enough” (see post #4 here) that you would also consider to be “safe enough” and is in a medium firmness range or firmer would generally be a suitable choice for a child. The choice between different mattresses that meet these criteria would really be more of a budget and preference choice than a “better/worse” choice. I would avoid memory foam mattresses for younger children because it may be too soft for children, it may not allow for free movement on the mattress, it is generally less breathable than other types of foam, and because the odds are higher that some children may be sensitive to some types of memory foam.
I would keep in mind that a mattress that would be “comfortable” for most adults would likely be too soft to be suitable for a child. You can also add a softer topper to any firmer childrens’ mattress when they get older and develop more adult proportions if and when they need a softer comfort layer in their “sleeping system”.
Phoenix