Hi rx135,
I’ve deleted your other post (which was a duplicate) and your PM asking for a reply to your mattress forum posts.
In almost 2 years on the forum I don’t think I’ve ever knowingly forgotten someone’s post (so the odds are exceptionally high that I will reply as soon as I am able to do so) but with only two hands and one head … and many demands on my time … it’s not always possible to reply within several hours although it would be very rare that I wasn’t able to reply within 24 hours 
I think the first suggestion I would have is to follow the steps and links to information in post #1 here. These steps and the other linked information in the post will greatly increase your chances of finding a mattress that is not only suitable for your needs and preferences but has the best possible quality and value.
I would also put my initial focus on local manufacturers and better sleep shops who are much more knowledgeable about their mattresses, transparent about the materials that are in them, and will put your interests above their own (instead of selling you “anything they can convince you to buy”). An online purchase can be very risky unless you are dealing with a manufacturer or retailer that is knowledgeable and transparent about what is in the mattresses they sell and can help you decide on a mattress that is suitable for your needs and preferences without having tested it in person. This is a fairly specialized skill. The salespeople at Costco know very little about mattresses and you would be completely on your own in terms of knowing whether what you are buying may be suitable for you or has good quality and value. The first guideline in one of the links in the post I mentioned earlier is also to avoid major brands completely because it would be very rare that they will disclose what is in their mattress and give you the ability to make meaningful comparisons with other mattresses. You would be rolling the dice unless you know exactly what you want and have good knowledge about the materials in any mattress you are considering.
Some of the better options or possibilities available to you in Houston are listed in post #2 here.
If you look at the layers of the mattress … you will see thick layers of polyfoam (such as Hypersoft, Supersoft Sealy foam, Sealy foam, and even “high performance” Sealy foam) which are all words for polyfoam. Most of this is lower density. These foams in the lower densities that Sealy uses (typically in a range from under 1 lb/cu ft to in rare cases 1.8 lbs/cu ft) are all much less durable materials than other better quality foams. For example you can see here that even the Sealy HD polyfoams range from 1.35 lbs to 1.8 lbs and these are some of their better quality foams not the lower quailty ones that aren’t HD foams that are even more common. Typically you will find these foams are in the 1.2 to 1.5 lb range.
Even more important though is the other side of your question which is …
How can I know for certain this mattress and the quality of the materials in it justifies the cost and how can I make meaningful comparisons to other mattresses?
The answer to this is by knowing the details of every layer and component in your mattress … especially in the upper layers which will soften and break down more quickly than the deeper layers of the mattress. I would never buy a mattress where you don’t know the specific quality of the foams used in every layer because if they aren’t listed it is much safer to assume that they are low quality and pass them by. High quality foams and low quality foams can feel the same in a showroom … but the lower quality/density foams just won’t last as long until the mattress loses the comfort and support that are the two main functions of a mattress. The loss of comfort and support also isn’t covered by warranties which are mostly meaningless as a way protect you from the main reason people need to replace a mattress and are more of a marketing tool to justify higher prices. The only way to get any sense of how long a mattress will last for you is to know the quality of every layer in your mattress so you can identify any weak links and without this information I wouldn’t ever consider buying the mattress.
Again … I would never consider a mattress where the quality of every layer wasn’t known to you and they don’t provide this specific information with their major brand mattresses (and most other retailers don’t either).
While these wouldn’t be your “final” option (there are others in the Houston list) … they are certainly one of your better ones. You would be dealing with people who understand mattresses and mattress construction and could provide more meaningful answers to your questions about their mattresses.
Most of the latex you will find has been certified by Oeko-Tex standard 100 class 1 (safe for babies) or other equally valid certifications for safety and IMO would make a very good choice for a mattress that was used by young children. It also doesn’t have the same risk either of offgassing or the slow recovery nature of memory foam which I believe is not suitable for yourn children. Some of the better information in the forum about children and mattress materials are included in the links in post #2 here.
Post #2 here has more information about the many factors that can affect sleeping temperature which should be helpful. While the foam in a mattress is only one of the factors involved in temperature regulation … latex as a foam category is generally the coolest of the different types of foam (memory foam, polyfoam, latex).
If you do decide to go in the direction of an online purchase … then the manufacturers listed in post #21 here are among the better choices I’m aware of and can act as “value reference points” for a local purchase but I would keep in mind that there is always some degree of greater risk involved in purchasing a mattress you have never tried.
I hope this helps answer your questions but if not feel free to post any others on the forum. Just keep in mind that they may not always be answered in just a few hours and that duplicate posts or private messages (unless for some reason their content wouldn’t be suitable for the forum) may take from the time I have available to answer forum posts.
Phoenix