Kids' Last Tour

From the introduction:

Grandpa is dead. It wasn’t a surprise, he’d spent the last years of his life preparing everyone for it. Then one day, he didn’t wake up. After burying him, you got your gear together, loaded up what food there was, and headed out. You have a couple of weapons, not everyone carries one. You’ve been outside the bunker before, gathering what you could from the ruins of the city. But now...

The city stretches to the horizon, and is made up many levels, hundreds of meters apart. You’re on one of the lower ones. You can see the sky occasionally, and it’s snowing. Without Grandpa, the bunker’s machines broke down and you have to find someplace new to live in.