Hi mjosta:
There is more information in post #2 here about mattresses for children. There are many links describing “safe” materials including a link to some additional comments and some general firmness guidelines for children in post #2 here. It’s quite a bit of information but it covers everything you would want to know about your question.
I’m not sure what you’ve read since you found the site but just in case you haven’t read it yet … the first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones. I know you’re limited by the choices being placed in front of you, but these guidelines will still be assistive for you.
While I don’t have the ability to maintain a complete list of the items carried at any particular point in time at Sleepy’s, the brands the carry generally don’t meet the durability guidelines recommended, even for growing children. So, in this case I’d tell you to use the mattress shopping tutorial and the durability guidelines in the aforementioned post about mattresses for children to make your best possible choice of the items presented to you.
Phoenix