Instructions
Have a kid with one of your spouses in game. You can have several spouses and several kids, but if you want to make them grow up then you need to have as many, or as few, kids as you want to age before you begin either of the tricks.
Sleep for roughly 1 week in-game. While only a week has gone by, to keep things interesting, the game will age your baby to a toddler, or from a toddler to an older child. You can sleep longer, if you're willing to spend more time to make your offspring age more.
Take up a long-reaching quest. The Crucible quest, for example, takes 10 years in-game. Those 10 years affects you, your spouse and your offspring, making them grow up from the beginning of the quest to the end. The Spire quest is another quest that takes time, which will cause your kids to grow up in your absence while you're off being a hero.