Hi imagerockone,
Welcome to the Mattress Forum! 
There are some general differences between a latex support core and an innerspring support core in this post here. Both would be a good choice for a support core for a growing child.
Both of the items you are considering use good quality materials and would be appropriate for a child. There are some general guidelines for children in post #2 here. A “medium” to “medium-firm” comfort level is generally recommended for children of that age group. The choice would come down to your own personal preferences and personal value equation.
Both Sleep on Latex and My Green Mattress are members of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. They are extremely knowledgeable about latex and different configurations, and I would not hesitate to recommend them for your consideration for a mattress for your child. If you do decide to purchase from one of these site members, be sure to avail yourself of your TMU discount code of MAYUNDR for My Green Mattress.
Because Sleep On Latex protects itself against the code harvesters you can request the code in a Private Message or email. Alternatively you can always place your order over the phone (312.487.2865) and mention that you are a member of The Mattress Underground to receive your discount.
If you have more specific questions, I’ll do my best to be assistive.
Phoenix