Flashbacks would makes sense for IT: Chapter 2 on a couple of levels.
Then there is the fact that in adapting Stephen King’s massive tome, there are still parts that have been left out.
Flashbacks would also help to flesh out the mythology while giving audiences more of the kids they fell in love with in the first film.
Even though audiences know that IT: Chapter 2 is a sequel and these two films are connected, flashing back to the kids will help cement that by tying together the two time periods.