Hi Samsmum,
You can see some comments about Restopedic in this topic and this topic and this topic and a forum search on Restopedic (you can just click the link) will bring up more comments and feedback about them as well… They tend to use higher quality materials than most of their competitors in similar budget ranges and they certainly make some very high quality/value mattresses.
I don’t have any personal experience with any of their mattresses so they would be the best source of guidance about which of their mattresses would be a suitable firmness choice for a child but I wouldn’t have any concerns about the quality or durability of the materials in any of their mattresses.
The choice of materials and components would be a preference choice but if you are comfortable with a more traditional innerspring mattress with polyfoam comfort layers I would probably tend to lean towards one of their lower budget two sided innerspring mattresses that are in the firmer range and then when your child gets older and their body starts to develop if they need some additional comfort or pressure relief it would be a good “base mattress” for adding a topper.
Phoenix