Hi torsi9,
Congratulations on your new mattress 
Latex comes in a very wide range of firmness levels from ultra soft to ultra firm so it may be that the latex you tried was softer than you prefer. It also has a somewhat different feel because even in the firmer versions it is very elastic and contours better to your body profile than polyfoam and to some people this can “translate” into a perception of softness (which can be very subjective).
Post #2 here includes links to some of the better forum posts about children and mattresses and they also include links to mattresses that would be well worth considering.
A thinner layer of firmer latex can be one of the better choices for a child.
I fixed the SleepEz link which I had linked to the Oeko-Tex site in error. It’s amazing that you were the first to point this out when the post is almost 2 years old :). Thanks for bringing it to my attention.
My Green Mattress recently changed their website and now has several versions of the Pure Echo and since they also changed the URL I’ve also corrected that link as well. Thank you once again.
With a child’s mattress an online purchase can make more sense because they can’t test the mattress and provide you with meaningful feedback anyway but this would depend on your comfort level with an online purchase, your conversations with the online retailers and manufacturers you are considering, and on how your online options compared with what you have available locally.
Phoenix