Good question. I wasn't sure either, but I googled, and not suprisingly, google led me right back to a blog here on FindAnyFloor.com.
blogs.findanyfloor.com/consumer/Ru...These guys seems to have covered just about every flooring topic!
Anyways, it says that foam flooring, which is made of EVA foam is safe for kids - it is non-toxic, lead free, PVC free, and latex free. I'd say it sounds like a great choice for your son's playroom.