Hi Syracusa,
The reason for adding a topper (or to use multiple layers of the same material in a mattress) is to change the feel and performance so it’s a better match for someone in terms of PPP. There are many latex mattresses that use multiple layers of the same material for this reason and it certainly wouldn’t be pointless if some additional softness or pressure relief was important. Having said that … given the feedback you’ve had from your kids so far I would hesitate to make any changes. A softer sleeping surface may not be in their best interests (just like using too much sugar in a diet may be something they would be attracted to or “like” but may not be in their best interests).
Foam toppers (either memory foam, polyfoam, or latex foam) in a suitable thickness and softness (lower ILD) would all add additional softness and additional pressure relief that would generally be more effective than fibers which tend to compress and become firmer as they pack down.
There are also many variables that work together to affect the sleeping temperature of a mattress outside of just the type of foam that is used although latex foam is generally more breathable than polyfoam which in turn tends to be more breathable than memory foam so the type of foam you use can certainly make a difference. There is more about the many factors that work together to affect the sleeping temperature of a mattress (including the softness of the mattress, the type of cover and quilting, any mattress protector or mattress pad you use, your sheets and bedding, and the bedclothes you wear) in post #2 here and the posts it links to and as you can see the type and breathability of the upper layers of foam are only one of the variables involved.
Phoenix