Hi leol,
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.
I would avoid soft for a child and medium would be a more “average” choice for a child as well as a guest bedroom.
A mattress is soft enough that it can “squish” a little bit even if it’s 39" wide it would probably be fine although if it sits too deeply inside the bedframe rather than more on top of the bedframe then it may be a little more difficult to tuck the sheets underneath it.
You can see a description of how they manufacture the mattress here. Blended Talalay latex is among the most durable of all the foam materials so it would certainly be a very durable choice that would last for many years.
Phoenix