That's pretty much it. I realize this might not be the best subreddit, but I've tried in a number of more bible-related places and haven't had much luck.
The Bible itself doesn't say. I'm hoping for some outside source (even if that source is well-reasoned speculation).
Children, to be sure.
Pharaoh, in the narrative world of the text, was likely a first born--but he wasn't killed by the Destroyer.
Also, check out Propp's commentary on Exodus in the Anchor Bible Series for more, good information.