Hi deighton,
The first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best possible choices.
Post #2 here also includes a list of the better forum posts about mattresses and children and includes links to some very good options for some good quality and durable children’s mattresses.
Almost every material will have some degree of offgassing but the real issue is the “safety” of the offgassing. Most of the better mattresses that use North American polyfoam or memory foam that is CertiPur certified so any offgassing would be within the limits that most people would consider to be safe in terms of harmful substances or VOC’s. Most of the latex you encounter will be Oeko-Tex certified which is a more stringent testing protocol yet and latex would also be a material that most people would consider to be “safe”. If you also use a “safe” fire barrier such as wool or a viscose/silica blend and any glue in the mattress is water based then most people would be quite comfortable with the “safety” of their mattress although some people who are more sensitive or have certain health conditions (such as multiple chemical sensitivities) or for personal reasons may choose to avoid synthetic materials completely. Each person may have different criteria though and for those who want to take the time to do the detailed research into a very complex subject and the often contradictory and misleading information that surrounds this issue then post #2 here and the posts and information it links to would be a good place to start that can help you answer the question of “how safe is safe enough for me?”.
Dreamfoam is one of the online manufacturers that are a member of the site and like the other members here compete well with the best in the industry in terms of quality and value. The mattress you are looking at though is not an “all latex” mattress but a latex/polyfoam hybrid. It has a 3" layer of latex in the comfort layer. It’s unusual to find a latex/polyfoam hybrid in the lower budget range you are looking at because latex is a more costly material than either polyfoam or innersprings. They also carry an innerspring mattress which has some latex in the comfort layer. In most cases the support core of a mattress isn’t the weak link of a mattress in terms of durability but to the degree the support materials will affect the useful life of a mattress latex is a more durable material than either a pocket coil or polyfoam. The most important part of durability is the quality of the materials in the top few inches of the mattress because a mattress will usually soften and break down from the top down. There is more about the factors that can affect durability in post 4# here.
Phoenix