Hi jbrady3324,
The DIY in this case is a good way to go to get the best of all worlds. While a soft Dunlop layer will feel a bit firmer than Talalay in the same thickness and ILD. I do believe that a soft sleeping surface for 3 years old has a good chance to compromise alignment, reduce the blood supply, and impair the development and function of the epiphysial plates in a child.
The Merck Manual for Medical Information has a section on Causes of Bone Disorders in Children which" typically involve the gradual misalignment of bones, which is caused by forces exerted on the growth plates as children are developing. A poor blood supply can also damage the growth plate, as can separation from the rest of the bone or even minor misalignment. Damage to the growth plate suppresses the growth of bones, distorts the joint, and can cause long-lasting joint damage (arthritis)."
Unless otherwise advised by a medical professional, I’d err on the side of caution and select the firmer version side up if you chose to go with this dominating layer alternative (The “dominating layer” terminology I used refers to the practice of using “firmer” layers on top of “softer” layers).
From a suitability perspective, this is a more unusual design approach that has firmer materials on top and then changes the firmness of the deeper layers to add additional “give” underneath the latex. This would be similar to a “dominating layer” ( see post #33 here and the posts it links to) These types of designs create a firmer sleeping surface with a much more “on the mattress” feel with some additional give and softness underneath the firmer layers. Out of all options you mentioned this would be best. It also offers the advantage that you can assess your daughter’s alignment while she sleeps and if any slight misalignment is observed then you can move the 2" soft layer to the bottom.
I’m looking forward to finding out what you end up deciding.
Phoenix