There are a few scenes where we see the brute force capacity of "It". It tears Georgie's arm off, It slashes the chubby kid's gut, and in the final scene It throws one of them against the wall. According to the news clippings It took down a camp of ninety adults single handed. It seems to be pretty good at luring the kids away from the group and also has apparently unfettered access to their homes. I can appreciate that It 'feeds on their fear' so It ideally wants to keep them alive for optimal feeding but once It has identified them as threats why wouldn't It just isolate and quickly kill them Itself (rather than make a half-baked attempt to get the school bully to do it). Even in the final battle scene when it's quite obvious that these kids are a threat, It continues to resort to scare tactics rather than slice and dice. I get that this is fantasy so I am mostly wondering if there are some subtleties about the nature of It from other sources that might explain how It fumbled such an obvious upper hand.
I'm not sure if this answers your question, but I hope it helps a little. The creature known as IT is a shapeshifter that usually takes the shape of something the person its "hunting" fears. The drawback to this ability is that IT also takes on most of the "fear's" characteristics, and is bound by that shapes "laws of nature" (I.e. a werewolf can be harmed by silver, a ghost can pass through walls, a vampire can be harmed by sunlight or a stake through the heart.....). In the television miniseries the kids convince IT that the inhaler one of the kids use is full of acid, and they spray it in IT's clown form face. Being a "human" clown, the laws of nature dictate that acid can hurt IT, so IT's face gets burned.