Hi dnamem,
Post #2 here has links to some of the better posts and threads around the forum about children and mattresses.
I think the best source of information about comfort levels would be a phone call to the manufacturer themselves who has much more knowledge and experience with their mattresses than anyone else (including me) and would be the best source of guidance for which of their mattresses would make the best choice.
In general terms a medium/firm to firm choice (of the comfort layer not the mattress overall) would make the best choice for a child.
Sometime around the age of puberty when their bodies become more developed and “curvy” and they begin to indicate either through how they are sleeping or by making comments about it that they need some extra softness because their mattress is becoming less comfortable. In some cases they may not need any extra softness at all depending on their body type and sleeping position and the specifics of the mattress. If the mattress is still in good condition and has no soft spots or “hills and valleys” and has an even firmness across the surface then a topper can work well. If there are soft spots or dips in the mattress then a topper wouldn’t be suitable because it will just “follow” the soft spots or dips underneath it.
Phoenix