Other forms: kittenishly
Someone who's kittenish is fun-loving and playful, but also a little flirtatious. Your sister, although she's an adult, might jump up and down clapping her hands in a kittenish way whenever she wins a game.
Many behaviors that are common in children are considered kittenish in grown ups, especially women. A kittenish spirit might be part of an actress's signature style, for example, and you could also call kittenish behavior coquettish or coy. The word stems from the idea "playful as a kitten," and kitten is from the Old French chitoun, "little cat."