Hi junogirls,
In case you haven’t seen it or it would be valuable for you … post #2 here has links to most of the better posts and threads about children in the forum including links to many good choices.
Depending on the age and the size of the child … generally a minimum of “medium” and up towards firmer from there so that there is only a little bit of sinking in. Not quite as firm as a baby who need the firmest surface. As they get older and begin to develop more their needs would start to shift towards the needs and preferences of an adult. I would keep any soft foam layers to no more than an inch or two at most. Some foam mattresses may have enough softness in the top of the core material itself to not need any additional soft layers.
I would also avoid memory foam (both because of softness and possible sensitivities) with children who are still growing and the “gel foam” is probably memory foam.
Unfortunately … neither Kingsdown or most chain stores will disclose what is in the mattresses they sell with any detail so there is no way to either compare or evaluate them … and I would tend to avoid them completely.
Post #3 here has my thoughts about the Ikea mattresses but I think the Ikea would be a much better choice of the ones you are looking at and has much higher quality, more natural, and more breathable materials and is better value. A slightly thicker mattress pad can always be added to it if they need a bit of extra softness although Ikea rates all of its mattresses as a variation of “firm” and with the natural elasticity of latex it may be fine by itself.
Phoenix