Hi diesiren,
You can read about the factors that are involved in durability and the useful life of a mattress in post #4 here and the other posts it links to. Innersprings are not normally the weak link of a mattress and the most important part of durability is the materials above the spring. The main reason that so many innerspring mattresses have a shorter lifetime is because of the quality of the foam above them which can soften and break down long before most innersprings … not because of the innerspring itself … although with some types of innersprings (particularly with higher gauge pocket coils that use thinner wire) that have thinner layers of foam above the springs so that the springs are compressed more deeply then the springs themselves can be a durability factor as well.
I tend to resist the temptation to attach a specific number of years to any mattress because there are so many factors involved in durability and the useful life of a mattress. Some latex mattresses will last less than 20 years and some will last more depending on the type and firmness of the latex and on the amount of softening that it would take to put someone outside the range of their comfort and support needs. Some people may have “room” for more foam softening and the same mattress may no longer provide the comfort and support that one person needs but may still be fine for someone else that has a different range for comfort and support that is suitable for their body type and sleeping positions (see post #2 here). With the age and smaller body type of a child I would say the odds are high that the mattress will last until they go to college but they may want to do some fine tuning (such as adding a topper) once their body starts to mature and develop as they may need some additional softness on top.
The 150 lb weight that they mention is not because of durability issues but because a 6" latex mattress may not be as comfortable or as suitable for a larger body type. It can also be flipped and rotated so this adds to the durability of any mattress even that is made with as durable a material as latex.
The “regular” mattress (assuming you mean this one) appears to use the same thickness and type of latex (100% natural Dunlop) but many manufacturers have a smaller margin for mattresses that they market as “children’s mattresses”. There may also be some difference in the type of cover or quilting they use but they should be able to tell you if there are any other differences that account for the significant difference in price between them (and it would be interesting if you posted their reply here for others that are looking at the same choices). Once again any weight guidelines would be for “guestimates” for suitability not for the durability of the latex.
Phoenix