Please do not buy an IKEA mattress. We had a horrible experience with one of their children's mattresses stinking for WEEKS after we bought it. Finally returned it but it was a hassle.
We are replacing it with the Beautyrest Beginnings mattress, which was more expensive, but not as much as Naturepedic. We are buying a twin, though, so it is cheaper. A lot of reviewers said it had no smell at all, it's low VOC, and doesn't use phthalates, formaldehyde, methylene chloride, CFCs, PBDEs, mercury, lead and heavy metals. It is "Certi-PUR US certified" for whatever that's worth.
They have a pillowtop which seemed excessive for a 2 year old used to a crib mattress, but I wanted something comfortable enough for Mom and Dad, too.
http://www.landofnod.com/twin-simmons-beautyrest-beginnings-plush-mattress/s300829 I get it next week, so hopefully we will like it better than the IKEA one!