Hi Lnr103,
Since you have already purchased from them it may be well worth considering their latex children’s mattress here.
Other than that … post #2 here and the topics it links to also includes much 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 include many good options for children in a wide range of prices.
Phoenix