Children over the age of 6 can be sent to be raised by a guardian. This can be pretty much anyone, a vassal or courtier, or a foreigner. The child will tend to get the traits, skills and education of their guardian, as well as his or her culture and religion. You can also suggest a guardian for children in other courts, meaning that you can mold the heirs of your vassals to your liking, provided they like you enough to agree. Children in a foreign court serve as a kind of hostage, since they can be imprisoned and executed freely should you attack their guardian's ruler.