No washing will remove the chemicals. It may help break down some portion, but it will not remove them entirely.
Sorry to be a bummer, but there is no way to totally remove the chemicals, since the fibers of the threads are pre-treated before the material is woven, much less before the garment is made.
If you're wanting to try to break down the chemicals as much as possible, then use fabric softener and a soap, not a detergent. There's a difference.
I would use Dawn dish liquid (the blue concentrated kind), and fabric softener. Be sure to run lots of rinses after.
But truth be told, I just don't buy those PJs, since it is not possible to guarantee that all the chemicals have been removed, whether it be from purposeful laundering, or from years of previous wear. Those chems cling hardcore.
I have, however, used the Dawn/softener approach with carseat covers and other furniture-type material that's been treated and is unavoidable (unlike the PJs, which are).
Never wash a carseat cover in hot, or dry in the dryer though, as you don't want to risk shrinking obviously.
Anyway if you were to use some enzyme-based cleaner (like Bac-Out) on pajamas or any clothing though, be sure to wash/rinse in hot water after. Otherwise the enzymes can remain on the fabric and cause skin irritation.
If you're wanting warm PJs go for layering and designated playwear fleece rather than designated sleepwear.