>>11539414
-urgent sequel plot involves young children
-movie wasn't already in production immediately after the first movie, which is already a year or two old by the time 1st movie is released.
-movie is a hit, let's do a sequel!
-time passes as they write and re-write sequel...
-whoops! children grew up!

see: Wednesday, Stranger Things,

(the Duplo thing at the end was just a horror-movie style joke punch. you weren't really supposed to expect a sequel out of it. like when the monster reaches his hand out of the ground or whatever. it implies a sequel, but it's not really the start of it)