Yes, diet is important. Feed young kids jerky and other things that take some effort to chew. It makes the jaw grow so there will be more room.

Another thing told to me by a mechanical engineer who is the son of a dentist: people from different countries can have different characteristics and inter-breeding may cause problems. He said oh, German teeth in an Irish mouth - the kid in question is indeed a mix from those two places. My kids ortho said they do things differently depending on race too. Example they gave was that Chinese teeth tend to be more straight across the front as opposed to more arched, so they take that into account when doing braces.