Trivia: In order to show that Eddie has severe hang-ups about his relationship with his mother, both his mother (in flashbacks) and his wife (briefly seen in the opening) are played by the same actress, Molly Atkinson, albeit in significantly different makeup and wardrobe to make it subtler.
Trivia: Bill Skarsgård has a lazy eye that he was able to train himself to focus in the same direction as his other eye. Thus, when Pennywise's eyes look in different directions, it's not an effect... Skarsgård is just letting his lazy eye naturally drift. His co-star Billy Hader didn't realise this and asked Skarsgård how the effect of his eyes focusing in different directions was achieved, assuming it was CGI. Skarsgård replied, "Oh, you mean this?" and let his drift sideways, horrifying Hader.
Answer: I think it's more of an attempt at a silly joke, juxtaposing the insane imagery with a tender song. But I've also seen the suggestion that it was an Easter Egg/reference to the book "The Langoliers," in which the song is mentioned. (And given the "It" films have some general Stephen King Easter Eggs referencing things from his other books, that makes sense).
TedStixon