Hi Kazhuveri,
Welcome to the Mattress UnderGround.
Given that your child is 100 lbs now and may reach 140 lbs, a 9-inch medium-firm latex mattress might be a better long-term option, as it will offer better durability and comfort as they grow. The 6-inch mattress with ILD 29 could be fine for now but might not last as long or offer as much comfort as they get heavier.
If you are hesitant about Foam Factory due to their reviews,I would suggest @Latex_Mattress_Fact1 or @Arizona_Premium. These manufacturer options should offer you peace of mind with both quality and longevity, ensuring that the mattress can be passed down to younger siblings as well. Not all dunlop or talalay latex are created equally. Just as with any other product, there are quality control measures that can set one manufacturer apart from another. I am not as familiar with mattresses247, but if you have been happy with them and they offer exceptional customer service, there is not reason not to pursue them again. Of course, all of the Trusted Member Manufacturers have been scrutinized here for their reputation, quality and customer service.
Most Dunlop comes out of Sri Lanka, and Most Talalay is produced in the USA or the Netherlands (the better quality controlled latex, that is).
This thread speaks to a crib mattress on up, so there is an interesting conversation there.
Regarding plywood. A plywood base will work well to support a latex mattress, as it provides a firm, even surface. Latex mattresses are typically flexible and breathable, so a solid, flat base (like plywood) is perfect for ensuring proper support. Make sure the plywood is well-ventilated to avoid any mold or mildew issues over time. You may even want to place a coir or hypervent layer between the mattress and the wood. Additionally, be certain the plywood is not warped and is thick enough to support the mattress and the sleeper. If the difference between 3/4" and 1" is minimal, go with the 1".
Hope this helps,
Maverick
Just keep in mind, this mattress is for your child, not yourself. An adults needs are somewhat different than a child’s needs. Latex is certainly a durable long lasting foam. You should also consider a full size mattress rather than a twin, if room permits, your teen will appreciate you as they grow.