Hi viva_dc,
I think if you were to call 3 other manufacturers you may have an additional 3 different opinions because there are so many variables involved that you aren’t likely to find any specific consensus and this isn’t an exact science.
Since you asked … I’ll throw my own thoughts into the ring as well but I wouldn’t put more weight on it than your other suggestions because the truth is there isn’t one “perfect” suggestion that would be the “best” for any situation and many combinations may work equally well. The only thing I would be cautious about is a mattress that is too soft for children because they tend to need firmer and more “supportive” mattresses than adults that have more developed and “curvy” bodies.
I would tend to use thicker layers of latex than 3" because it would be more suitable as your child gets older and heavier because thicker mattresses can “adapt” to different body types and sleeping positions more effectively than thinner mattresses or layers. There is more about the effect of thickness in post #14 here.
If you are looking at a DIY construction and are limited to about 5" of latex then you can choose a combination of layer thicknesses that add up to any height you wish (such as a 3" layer with a 2" layer on top of it). Some of the better sources for latex layers I’m aware of are listed in post #4 here.
I would also consider using less wool unless you have a specific reason for using both wool batting inside the mattress (which may shift or get “lumpy” over time) and an additional wool topper on top of the mattress. For most people a latex mattress with a wool quilted cover would allow the latex to contour more effectively and still provide the temperature regulating properties of wool. If you prefer a mattress that uses thicker layers of wool for the sleeping surface (which would also be suitable for a child because wool tends to compress and become firmer than softer foam) then I would build a latex mattress with a thin stretch knit cover and then add a 2" or 3" wool topper on top of it.
If you were to use a single 3" layer I would probably use something in the medium to medium firm range (about 28 - 32 ILD) because lighter body weights will compress the latex less and a medium layer would be firmer for a child than it would be for an adult with a heavier body weight. Thinner mattresses will also tend to be firmer than thicker mattresses using the same material. Also in very general terms, girls will tend to prefer a little softer than boys (and boys will also tend to get heavier as they mature).
If you choose multiple layers then I would go with something in the firm range in a bottom 3" layer and then in the medium range for the top 2" layer.
The component post I linked also includes some good sources for covers.
There are two fire regulations that a mattress needs to pass before it can be sold in the US and both of them are national regulations not state regulations. The first is CFR 16 part 1632 (the cigarette smoldering test) and the second is CFR 16 part 1633 (the open flame test). Both of these require actually burning a prototype not putting together a mattress based on specs or individual components. If you are building your own mattress out of your own components and layers then it isn’t required to pass the fire regulations (which only apply to mattresses that are sold as a complete mattress and not individual components) but you would also have no way of knowing whether your mattress design would have passed the test.
Post #2 here also includes links to the better topics and posts in the forum about mattresses and children that includes many good sources for children’s mattresses and may be useful as well.
Phoenix