Hi moosemom,
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 mattress choices for children.
Both of the mattresses you are considering would certainly make good quality/value and suitable choices for a child and there are no lower quality materials or “weak links” in either of them.
You can see a description of their “B Stock” mattresses on their page here. They are new mattresses that have cosmetic defects and are sold at a steep discount because they don’t have any warranty or comfort exchange options and all sales are final.
Phoenix