Eh. It doesn't seem that unworkable? I haven't read the book in ages, and maybe it's different in the movie, but I think there's a lot of "Pennywise fucks with them" moments in canon that don't have Pennywise appear on page. I assume they just want, like, blood to start dripping out of the faucets and creepy old pictures to start moving and black balloons to follow them around.