An adorable child can be called a moppet. A sweet moppet sitting behind you on an airplane is completely different from a screaming, kicking kid occupying that seat.
Though it's a bit old fashioned, you can use the informal word moppet when you're talking fondly about a child. You might watch a group of moppets learning to skate on an ice rink or enjoy a movie about some adorable moppets who scheme to get their lost dog back. Traditionally, this word usually referred to a girl, and earlier it meant "a doll," from the Middle English moppe, "little child or baby doll."