The specs you listed don’t contain any meaningful information and are missing the foam densities which is the most important information that you need to make sure that there are no lower quality materials or weak links in a mattress.
I would also keep in mind that mattress mattress warranties only cover manufacturing defects and they don’t cover the gradual (or more rapid in the case of lower quality comfort layers) loss of comfort and support that comes from foam softening that is the main reason that most people will need to replace their mattress. In other words the length of mattress warranties have little to nothing to do with the durability or useful life of a mattress or how long it will be until you need to buy a new mattress. If there is an actual defect in the materials it will usually show up early in the life of the mattress (usually in the first year or two) but knowing the quality/density and durability of the materials in your mattress is always a much more reliable way to assess the relative durability and useful life of a mattress than the length of a warranty. There is more about mattress warranties in post #174 here.
[quote]We have a 14 year old son who is tall and growing…
And we have a 9 year old girl…
I’m guessing our son probably needs something equivalent to ours, however the 9 year old? I’m tempted to just go with the 8" mattress which is only about $181.
The thickness of a mattress is really just a side effect of the design and is only one of many variables that can affect the feel and performance of a mattress and by itself isn’t particularly meaningful (see post #2 here). In some cases higher weight ranges will sometimes do better with a mattress that is a little thicker than lower weight ranges but even this depends more on the specific design and combination of materials in the mattress and on how well the mattress “as a whole” matches your specific needs and preferences than anything else.
In other words … there isn’t a specific thickness that is inherently better or worse because it always depends on what is inside a mattress rather than on the thickness that all the individual materials and components in the mattress add up to.
For older children I would use similar guidelines as I would for adults in a similar weight range except I would look for a mattress in a medium to medium firm range (girls generally tend to prefer slightly softer than boys but this isn’t always the case). I would just avoid choosing a mattress that is too soft for growing bodies so I would avoid mattresses that have thicker or softer comfort layers. This way you can always add a softer topper if they need some additional softness and pressure relief once they reach their more adult proportions.
The choice between different types of materials and types of mattresses (see this article) is really more of a preference and budget choice than a “better/worse” choice although I would tend to avoid memory foam for growing children. Just like with adults I would also make sure that there are no lower quality materials or weak links in any mattress you are considering for an older child so that the mattress will maintain it’s comfort and support at least until they are ready to go out on their own.