IT Chapter Two isn't as good as the first movie, and one of the major reasons for that is how it uses the kids from 2017's IT. Once again directed by Andy Muschietti, IT Chapter 2 sees Pennywise return after 27 years away, once again coming to torment Derry and its inhabitants, which leads to the return of the Losers' Club.
Now fully-grown adults, IT Chapter Two finds that most of the Losers have moved on from Derry and mostly forgotten about what happened with Pennywise, until Mike Hanlon brings them back to fight him. The change of cast and scope was pre-ordained by the decision to split Stephen King's IT novel into two different movies, one telling the story of the Losers as kids, and one as adults. However, IT Chapter 2 brings back most of the cast from the first IT movie, but unfortunately, it's to its detriment.
Appearing through flashbacks, with some de-aging effects applied to try and make it look like no time has passed, IT Chapter Two's story hinges on the Losers as kids, and it becomes too much. A lot has been made of IT 2's epic runtime, and these constant flashbacks are a big part of that. While some provide a bit of new context, such as in the plotline regarding Richie's sexuality, many of them simply cover old ground. Because each member of the Losers has to find their own token, then it means each corresponding kid has to appear, and things start to get very repetitive pretty quickly. IT Chapter Two fails to give its adult meaty storylines that push their characters forward, and being so stuck in the past is a major cause of that issue.
Not only does having the kids so involved make IT Chapter 2 much longer than necessary and overly repetitive, but it also highlights larger issues with the film that might not have been so obvious otherwise. The young cast of IT is quite brilliant, and the adults of IT Chapter Two cannot compete. Where the former are funny, charismatic, and have a great chemistry, the adult cast - with the exception of Bill Hader and James Ransone - are much more stilted and given far less to do. This would have been a problem for IT Chapter 2 anyway, but it's made into a much starker contrast by having scenes regularly switch between the adults and the kids.
IT Chapter 2 bringing back its younger cast is an understandable decision, since the first movie was so popular, and the performances of Finn Wolfhard, Sophia Lillis et al were a big part of IT's success. However, it fails to blend the two together, which shows the dangers with splitting King's book into two such distinct parts (and, on a broader level, why IT is such a tricky novel to adapt as a whole). IT Chapter Two has a few different problems, including not being scary enough, its opening act being too brutal, and underusing Bill Skargård's Pennywise, but the one that hurts the movie the most is the over-reliance on and constant use of the kids.