Hi dhanley,
You may have read this already but post #2 here and the posts it links to have more information about mattresses and children and “safe” materials including a link to some general guidelines for children in post #2 here. It also includes a number of links to many of the better forum posts and topics about mattress and children as well which have more information about many good mattress choices for children although there would also be many others that would fit the same general guidelines as well.
The Natural Escape uses natural Talalay latex and would probably be a more “comfortable” choice if you are also going to be using the mattress from time to time because of the pocket coils and the latex. It also has firmer edge support that can help if you are sitting on the side of the mattress for “story time” but I would confirm that it was firm enough for a child (medium or firmer is usually the best choice for children). It’s also two sided which can add to the useful life of a mattress but of course it’s also a little more than the budget range you were looking at. The choice between an all latex mattress and a pocket coil latex hybrid would be a preference choice and not so much a “better/worse” choice because the weakest link of a mattress is generally in the comfort layers not the support materials or components. You can see some comments about how a latex support core compares to an innerspring or pocket coil support core in post #13 here.
As Joe mentioned the Q3 Jr Latex would also make a very suitable choice for a child. It uses blended Dunlop latex which like all latex is certainly a “safe” material (it’s Oeko-Tex certified) but it isn’t 100% natural if that’s an important criteria for you.
The reason the it may not be suitable for many adults wouldn’t be because of any durability issues but because it’s only 6" thick and this may not be thick enough and may be too firm for many adults unless they are in much lighter than average weight ranges and prefer a firm mattress (which means that they probably don’t sleep on their sides).
I should also comment that while wool can form visible impressions as it compresses (particularly if you tend to sleep in one area of the mattress) … this generally doesn’t affect support/alignment because wool becomes firmer as it compresses unlike foam materials which generally become softer over time which can lead to the loss of comfort and support. Having said that I don’t think that I would choose the Pure Echo for someone that was in a higher weight range because the wool could compress more into the coils over time even with a good insulator and this could become uncomfortable and could also become a warranty issue for higher weight ranges.
All of the mattresses you are considering along with many others listed in the children’s topics I linked would certainly make a very suitable choice for a child but most of the differences between them would be based on your own preferences, budget, and on lifestyle choices that are important to you regarding the use of natural or synthetic materials.
While there is no way to quantify how long any mattress will last for any particular person, if a mattress is well inside a suitable comfort/support range and isn’t close to the edge of being too soft when it is new and meets the minimum quality specs that are suggested in the guidelines here then it would be reasonable to expect a useful lifetime in the range of 7 - 10 years and with materials that are higher quality and more durable than the minimums like latex or higher density memory foam or polyfoam (in the comfort layers especially) or that are two sided it would likely be in the higher end of the range or even longer.
It’s always more realistic to think of about 10 years as a maximum reasonable expectation for any mattress no matter what the quality or durability of the materials and then treat any additional time after that as “bonus time” because after about 10 years the limiting factor in the useful life of a mattress will often be the changing needs and preferences of the person sleeping on the mattress (which would apply to a child as well as they grow older and begin to develop) and even if a mattress is still in good condition after a decade … a mattress that was suitable for someone 10 years earlier may not be the best “match” any longer.
Phoenix