A quick "google" shows several PC12's advertised for sale with 12 seats.
With one 1 year old infant, that would potentially make 13 "legal".
There was also a 3 year old, who if only a few months younger would also have been an infant.
Sorry if the post appears to be matter-of-fact, because I really do have sympathy for the loss of all on board, children and adults, but it does seem that IF this a/c had 12 seats, then overloading was very very marginal, and probably not the real issue.