Hi QTsleep,
All of the mattresses that you listed use high quality materials but as you probably read in the childrens topics I linked I would personally would tend to avoid memory foam for children’s mattresses.
I don’t know the criteria that are most important to you but I would also be aware that polyfoam isn’t an organic or even a natural material although by most people’s standards, if it was made in North America or was CertiPur certified, it would be a “safe” material. There is more information in post #2 here about organic, natural, safe, or green materials that can help you decide on the types of materials you are most comfortable with including in your child’s mattress. I also like wool and natural fibers on the sleeping surface for children either as part of a mattress itself in the quilting or added on top with a mattress pad or topper because it is a great temperature regulator and helps to maintain a drier and healthier sleeping environment and microclimate.
The Dormio Marriott Organic Mattress that you linked appears to use 6" of GOLS certified Dunlop (not 5") but I’m not clear on why there would be such a significant price difference between it and the Organic 6 which also includes 6" of organic Dunlop and is two sided yet sells for about $450 less.
Phoenix