I was never a fan of the Goosebumps books. By the time they were a big thing, I had already moved on to Stepehn King and other more adult versions of what Goosebumps was trying to do for kids.
Still, i found the movie lots of fun.
So, basically, a new group of kids (even younger then in the first film) find the book the heroes used to trap all the monsters in the first movie and open it up (they really should not have left that thing lying around).
Is Jack Black's R.L. Stine not in this one? I assume he's still providing the voice for Slappy.
We can't deduce a timeframe between the two films from the trailer apart from that the Stein house is empty and furniture-less, so it's possible the Steins have moved out. If Hannah won't age, it'd make sense they'd move around or Zach moves and the Stein's followed. It's possible something bad happened if the book was left behind.