Hi ladyfelina,
Welcome to the Mattress Forum!
There is 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 children which would be well worth considering. Any mattress that uses materials that are durable enough and are in a suitable firmness range (which these are) would be a good and logical choice for a child.
Their soft is a 28 ILD, the Medium is a 32 ILD and the Firm is a 36 ILD. The flippable Soft/Firm would feel “softer” with the Firm side up than the “all Firm” offering. When planning on keeping the mattress for a long time for a small growing child, the Soft/Firm combination is very popular. You’d use the Soft side initially, and as your child gains mass eventually transition over to the Firm side. If you don’t want a product using two different comforts, then the Medium can be a good choice that can last in comfort all through high school for your child. I don’t know that you would necessarily need to purchase a topper in the future. But you certainly could have that option.
Regardless, choosing something using high quality and durable materials like this and in the comfort range recommended for growing children (which these mattresses would be) would certainly be a choice worth considering. And as you’re aware, Arizona Premium is a site member here, which means I think highly of them, their advice and their products.
Phoenix