Hi.
Welcome to our Mattress Forum! 
These are all good things to consider when shopping for a child’s mattress, but I would point out that all materials including latex will have some degree of off-gassing (some of which can be smelled and some that can’t) but the important part is whether any off-gassing is harmful. Even food has off-gassing that can be smelled. In the case of latex the smell isn’t harmful. Talalay latex tends to have a sweeter smell and Dunlop tends to have a more rubbery smell but neither are harmful and both have been tested to Oeko-Tex standard 100 class 1 (safe for babies) standards. Both will dissipate to levels below normal detection. Generally speaking, any mattress that uses materials that are durable enough and are in a suitable firmness range and that has a reliable safety certification and you consider to be safe enough would be a suitable choice for a child regardless of whether it has an organic certification. I personally don’t have any concerns about any type or blend of latex in terms of its “safety”.
[quote] Currently I am considering:
7" Sleep on Latex mattress
EcoSleep by Arizona Mattress
kids mattress by Arizona Mattress [/quote]
You are choosing between good and good. …there are no weak links or lower quality materials in any of the two choices you listed and each could make a suitable and safe choice and both would fall in your budget range.
The 7" Pure Green is made with 100% Natural Dunlop Latex and organic cotton cover with quilted wool and zippered cover. The 6" EcoSleep for children is made with blended Talalay latex and uses what they call “fall off” which is strips of latex that are glued together to make a Twin or Full size latex core. They are using a water-based adhesive Greeguard Gold certified which is a reliable certification for VOCs and safe for children.
Generally, for growing children, you’d want something more in the “medium” to “medium-firm” range, due to their epiphyseal plate and postural formation, but other than this I would make your final choice based your preferences of materials, which one you believe would be most suitable for your children, your conversations with each manufacturer, the options you have to change the comfort level of the mattress when your children get older, their body matures and develops, and their needs and preferences change (by adding a topper, flipping the mattress, or changing the layering), your budget, and finally based on “best judgement” and which one you are “leaning towards” most strongly.
You may have read this already but just in case you haven’t … some more reading for your peace of mind…, there is more information in post #2 here and the topics it links to about mattresses and children and “suitable” and “safe” materials including a link to some general guidelines for children in post #2 here.
I hope this helps with your choice.
Phoenix