MASSIVE SPOILER in both BOOK and MINISERIES
Both basically say that because the story starts off with the Losers as adults getting phone calls giving them flashbacks to their childhoods and facing off against a killer shapeshifting clown creature, INDICATES THAT THE KIDS SURVIVED!!
So what is the point in watching the dull adult half with the clown if we already know what's going to happen?
At least the Muschietti movie adaptations did the right thing and tell the story fully chronologically, from childhood, before the Losers encounter Pennywise the Dancing Clown. It means that someone who has no knowledge of Stephen King's IT at all can watch the first movie and be in genuine suspense about whether or not all the kids survive...
...unlike the book and miniseries where they'll be given the biggest FU spoiler in horror story history from the start.
Don't you see how pointless the book and miniseries is in terms of its narrative structure now? Thank GOD the movie adaptation scriptwriters fixed this massive anomaly!