The deformed son was Beau (short for Beauregard) and what we saw of him, he seemed nice enough. Thaddeus is Frankenbaby (his official name in the credits is Infantata).
I truly doubt Travis is Constance's son. Not that I don't think the show would go there, I mean they've gone everywhere else so why not, but Constance made a point of saying Tate was her only "perfect child" and the others were damaged in some way. Travis doesn't seem to qualify.
I think the fourth child is still alive and long since bailed on Mommy Dearest and that whole circus. He/she will be integrated into the story at some point. Constance might not recognize him/her. Be highly suspicious of any new character in S2 who is blond, between the ages of 25 and 35, and has some kind of mental or physical handicap.
Or, it could have been a physical disfigurement that was taken care of by plastic surgery, which would explain things if Constance doesn't recognize her own child. Sometimes I wonder about Billie Dean...nah, couldn't be.