Hi kaforse,
Post #2 here includes links to most of the better posts and threads throughout the forum that talk about mattresses and children.
While the materials in Spaldin Mattresses in general and their Gemma Mattresses in particular are “safe” in terms of harmful substances and VOC’s (to their credit all their materials are Oeko-Tex certified as you mentioned) … I personally wouldn’t consider using memory foam in a mattress for babies or small children and I think there are many better options (including latex) available to you in similar or even lower price ranges than the Gemma.
I would seriously hesitate to pay $6000 for even a high quality latex mattress in king size for an adult … and there are many latex options available to you for a fraction of the price. All the latex you are likely to encounter whether it’s Talalay, Dunlop and made from synthetic latex, natural latex, or a blend is either certified through Oeko-Tex or Eco-Institut and would be “safe”.
I would also make sure you’ve read the tutorial post here which includes a link to several online manufacturers that are members here that sell a wide range of latex mattresses online and if you let me know your city or zip code I’d also be happy to let you know of any of the better options or possibilities I’m aware of in your area.
Hopefully it will also keep you out of the chain stores such as Sleepys and help you look in much better directions 
Phoenix