What is the reason my client’s son sees a “cat monster” and her daughter hears voices calling her name?

This, indeed, is an alien encounter that was masked through imposing an image of an animal. The son rightly sees this as sinister and that is why he depicts it as a monster and not a friendly creature inciting curiosity within. The daughter’s voice in the head experiences are an extraterrestrial psychic seeking to get her attention. This, too, is related to extraterrestrial encounters that have taken place.