I think you're right, but I don't know for sure. I'm not sure how Carters could get away w/o flame retardant when Hanna and Gymbo and LLBean have to treat their looser fitting and poly night wear.
And yes, sunshineandme, the mattress is almost certainly treated. Not much you can do now - the off-gassing has probably already taken place and it's probably less toxic now than it was new.