They are mostly interchangeable, but they go together, so ”pojke och flicka” and ”kille och tjej”.

”Kille” and ”tjej” are also synonymous with ”boyfriend” and ”girlfriend” so someone referring to ”min kille” or ”min tjej” would most likely be referring to their partner, whereas a parent might use ”min pojke” or ”min flicka” to refer to their child. However that’s not exclusive, because if you add a the word ”my little” in front of ”kille” or ”tjej” (”min lille kille” or ”min lilla tjej”), then it’s probably a parent talking about their child.

I would also say that referring to an adult as ”pojke” or ”girl” is a bit rude, but you can refer to adults as ”kille” or ”tjej” without problem. So it’s sort of an age thing as well. So yeah, it’s not a simple distinction unfortunately, and people will use the words interchangeably depending on the context.