Hi Rnagel,
There really isn’t an answer to this except to say “enough” to make a meaningful difference that most people would feel and that achieved the design goals of the mattress. Having an inch or so of latex buried deep inside a mattress wouldn’t make much difference for most people but having that same inch of latex as the top layer of a mattress would make a much bigger difference in some types of mattress designs. In general a couple of inches or more in the upper layers of a mattress closer to the top would be enough to make a difference in the feel and performance of a mattresses for most people but of course thicker layers of latex or an all latex mattress would have more of the benefits and overall “feel” of latex. This would be something like asking how much metal should be in a car relative to the other materials.
Soy foam is just regular polyfoam that has replaced a small part of the petrochemical polyols (one of the two main chemicals used to make polyfoam) with a soy based derivative. There is more about “so called” soy foams or other types of “biofoams” in post #2 here.
I think that deep, restful, restorative sleep is healthy but a mattress is just a means to that end so a mattress is only “healthy” to the degree that it contributes to high quality sleep. For that it needs to be a good match in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and needs to use “safe” materials. There is more about the factors involved in healthy sleeping as it relates to a mattress in post #4 here.
I wouldn’t consider any foam or mattress material to be “healthy” in the sense that it can help with specific health conditions but if polyfoam has been certified by CertiPur or other testing organizations which test for harmful substances and VOC’s then most people would consider it to be “safe enough”. There is more about “safe” materials in post #2 here and the posts it links to.
There are too many unknowns, variables, and personal preferences involved for anyone to recommend a specific mattress out of all the thousands that are available in the market but post #2 here includes links to most of the better forum topics about mattresses and children that include much more information about and sources for choosing a mattress for children.
If you are looking for a mattress that will last for 15 years then you would need to use very durable materials in the comfort layers especially. It’s also quite likely that when a child reaches their preteen and teenage years that their needs and preferences may change as their body develops and you may need to add a topper to their mattress to add additional pressure relief. A component mattress can also be worth considering since it allows you to replace or change individual layers in a mattress without having to replace the entire mattress if one layer softens or breaks down before the others of if their larger and more mature bodies need a different mattress than they did when they were younger.
Phoenix