"Would you be concerned if your son and DIL planned to keep a Pit Bull mix (now 6 mos. old) after finding they are expecting twins? "
Not if the dog was well trained, properly socialized, and had no history of aggressive behavior. My concerns would be the same regardless of breed. I think it is a horrible shame that Pitt Bulls (WONDERFUL dogs, IME) are given such a bad rap.... all because some nasty humans train them improperly/abuse them :(
Statistically small dogs are more likely to bite anyway.