Hi Davidson,
Post #2 here should be helpful and includes links to more information and some of the better posts and threads about about children and mattresses in the forum and more information about some of the better options for children’s mattresses I’m aware of.
Both of the mattresses you mentioned use high quality materials that would work well for a child but they are different from each other. the Naturepedic uses an innerspring and cotton batting while the My Green Mattress Echo uses an innerspring and cotton and wool and has the option of latex and a two sided construction and for most people would be better value out of the two (based on the materials inside it). My Green Mattress is a member of this site which means that I believe they compete well with the best in the industry in terms of quality, value, and service.
There is more about latex allergies in post #2 here but a latex allergy would be very uncommon in a mattress because there is no contact with the latex in the mattress so the only allergy of concern would be a type I allergy which would be very rare and would require significant precautions in many areas of life besides just a mattress because rubber is so common in our society.
There are many members here that have purchased a latex mattress for their children and been very happy with them. Latex is also very durable so when the time comes that children mature and begin to develop into adulthood and may need a softer sleeping surface the mattress will still be very likely to be in good condition without any significant softening or breakdown so you can just add a topper for any additional softness they may need and they will probably be fine until it’s time for them to leave home.
Phoenix