Hi Lnr103,
Blended Talalay latex is among the most durable foam materials in the industry so it would certainly be reasonable to expect that it would last for 10 years and probably longer.
There is more about the different types and blends of latex in post #6 here but all of the latex you are likely to encounter (either Dunlop or Talalay that is made with either natural or synthetic rubber or a blend of both) will have a reliable certification such as Oeko-Tex, Eco-Institut, or Greenguard Gold and based on actual testing I would consider any type or blend of latex to be a very “safe” material in terms of harmful substances and VOC’s.
The glue they use is also a safe material.
Overall it would certainly make a suitable choice for a child and it’s unlikely that you will find another all latex mattress or even a latex hybrid that uses meaningful amounts of latex in your budget range.
Phoenix