Hi sd213,
There are really two questions here that aren’t connected.
In terms of safety you would need to see the results of the actual testing for any harmful substances that can leach out of the materials and harmful VOC’s to make any specific comparisons between the materials used in any specific mattress and this isn’t information that would be available to you so there really isn’t any way to provide a specific answer. All you can know is whether a material is certified which means that it has tested somewhere below the threshold limits. The Oeko-Tex and Eco-Institut certifications that you will generally see for latex are a little more stringent than the CertiPUR certification for polyfoam but most people would consider any of these mattresses to be “safe enough” although some people are more comfortable avoiding polyfoam completely. In other words these are “best judgement” choices that depend on your comfort level with each type of material that is in a mattress. It would be a similar type of choice as the type of diet that your children eat where some parents would only consider organic food or even vegan food to be “safe enough” while others wouldn’t be as stringent in their choices.
In terms of durability there is also no way to quantify the number of years that any mattress will last but you can see the durability guidelines I would suggest in post #4 here. None of these mattresses have any lower quality materials or weak links in their design so I wouldn’t have any issue with the durability of any of them and it would be reasonable to expect any of them to last for 10 years or longer. The weakest link of a mattress is generally in the upper layers and synthetic Dunlop in the Freedom and the 100% natural Dunlop in the Roma and the UD natural are very durable materials. The Dreamfoam UD natural uses 2 lb polyfoam in the support core and the other two use latex so in “theory” the latex cores would be more durable but in practice the deeper layers will have much less effect on the useful life of a mattress than the upper layers (there is more about latex/polyfoam hybrids vs all latex mattresses in post #2 here).
This is something that only you can decide for your kids but I can confirm that children will generally do better with a mattress on the firmer side of medium rather than on the softer side until their body begins to develop into more adult proportions and weight when they can add a softer comfort layer if they need it (although some will still be fine with a firmer mattress depending on how they sleep).
Phoenix