So do any of your kiddos say "yes"?
A knows "no", has known and uses the "no" appropriately for a long time. Now J also seems to know the word "no" as well..... However I never hear A say "yes", if we offer something that we know he wants or an activity to do that he would be agreeable he never says "yes", he just repeats what we say.
I ask him to repeat "yes" from time to time and he can say the word and even does the ASL sign for it even though he isn't signing as much as he used to....but he doesn't use it in regular talking. Weird?