This is something I have thought about as well.
Their are two lines of thinking that i have found:
-First, In CoE you designate an heir, usually, but not necessarily one of your children. If you are that NPCs guardian you can train them in your guilds skills, and he/she will most likely join your guild before you ever die and start playing them. Back in the day it was expected for children to follow into their parents trade.
-Second, Caspian has stated that originally he did not intend for people to reincarnate as their heirs, and instead go from one life to another more at random (or rather placed by the story engine). You don't have to play your heir. You can reincarnate into someone in an entirely different community or guild and start a whole new adventure. So no you wouldn't be in the same guild.