This is from the instructions of I-485
you are not eligible for adjustment of status if any of the following apply to you:
You were admitted to the United States as a visitor under the Visa Waiver Program, unless you are applying because you are an immediate relative of a U.S. citizen (parent, spouse, widow, widower or unmarried child under 21 years old);
Your authorized stay expired before you filed this application;
You failed to maintain your nonimmigrant status, through no fault of your own or for technical reasons; unless you are applying because you are:
-- An immediate relative of a United States citizen (parent, spouse, widow, widower or unmarried child under 21 years old)
Going by the instructions and past experiences, it guess i would tell you that it should be okay in your case only when son becomes a USC... BUT you have to realize that they would be in an illegal status for a couple of years till the son gets his citizenship and they cannot work legally or leave the US and re-enter till then. It is a risk you and they should be willing to take. If the son's GC is denied, their hopes of legalizing their status would die away too.
My advise to you would be: Talk to a good immigration lawyer. We can only tell you what we think is right or wrong.