Hi williamng22,
Welcome to the Mattress Forum! … and I’m glad you found us 
The Tuft & Needle is a medium firm mattress and is CertiPUR certifed so it would certainly make a good choice for a child.
Post #2 here and the topics it links to also has much 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 the better forum posts and topics about mattress and children as well which also include more information about many good options for children which may also be worth considering as well.
I’m looking forward to finding out what you end up deciding.
Phoenix