Homebrew Campaign for Sons

I've started my 2nd homebrew campaign with my wife and sons (14, 12, 10 years old).  I'm having a block on how to progress the story. I initially wanted to start it with a continental event where the villains (Liches and Vampires) activate long-dormant dungeons of mostly Undead. But now I'm stuck on the WHY. I came up with the starting concept as an excuse for them to go through lots of dungeons. Unfortunately, when they're outside of a dungeon, I don't have a sense of purpose to drive the story.  

Any suggestions on what greater purpose liches and vampires could have for activating these dungeons? 

Thanks for any help.

They could be hidden Shadowfell or underworld temples containing undead legions that the liches and vampires want to activate, leading to zombie and other undead masses being able to escape and terrorize other towns. Another idea is that they contain old ritualistic items that they need for rituals or other such dark deeds. Sounds like a fun campaign though!

Possibly, they're trying to summon a god or construct of some kind, and there has to be mass bloodshed, so the party is going around trying to prevent these undead from escaping and causing an epidemic/apocalypse across the entire continent?

Ghostfire Gaming have a setting where the sun has been magically removed, an entire country plunged into external darkness, and as a result vampires managed to seize control. Shamelessly stealing that idea maybe the dungeons are all part of a continent size spell they’re trying to activate to black out the sun. To make it slightly more Freeform you can have that they need a certain amount of dungeons activated but not all of them, that way you don’t get the players saying “but we already stopped them” because they complete one dungeon but instead they’re still driven to keep racing to the dungeon 

1. Because that's what liches and vampires are wont to do. It's their reason fort existing. They don't need a greater reason.
2. Because it's a religious thing. They do this because the dogma of their religion call for it. 'Their god(s) say so'.
3. It's a philosophical thing. Life and Undeath are two sides of the same coin and the dictates of their philosophical beliefs are that there ought to be an equal number of living as undead.
4. It's a warped version of an earnest attempt to do good. Undeath offers the chance to avoid being blinked out of existence. These undead know that there is no afterlife, and so by creating more undead, they allow bodies to keep functioning and remain in the world beyond their expiration. 
5. It's only recycling. The bodies weren't getting used anyway, so why have skeletons and corpses littering crypts and dungeons when you can use them for your army, or to construct something?
6. The vampies/lichs need a workforce: as an army, to construct a monument; to protect the sacred item that keeps them 'alive'; to spread their religious message...
7. The vampires/liches are on a fetch quest and need several items from inside these dungeons. Activating the undead is just a side effect.
8. Having lost their memories of which dungeon a particular loved one is buried the lead lich needs to recover their loved one's remains in order to bring them back to 'life' as a lich.

That's all just off the top of my head. Remember though, that sometimes a moustache twirling villain is just doing the thing because they are evil and that can sometimes be enough. 

Vampires and liches both subsist on the essence of mortal beings. So, maybe they're raising an army to control and defeat the humanoid kingdoms so they can suck their blood and eat their souls. Or like others said, it's some kind of ritual, and it drains the blood/takes the souls of the whole continent. Liches and vampires are often also motivated by dominion over others, so perhaps it is to take control and rule for all eternity. That kind of thing.