noun
good child; well-behaved child
Affectionate term for a child who is obedient, well-mannered, or acting maturely. Often used by parents or caregivers when praising a child.
お利口さんにしてたら、お菓子をあげるよ。
If you're a good kid, I'll give you a treat.
今日はちゃんと挨拶できて、お利口さんだったね。
You greeted properly today—what a good child you were.
利口 is the base adjective meaning 'clever' or 'sensible', while お利口さん is a noun phrase with the honorific お and the suffix さん, used specifically for children.
Formed from the adjective 利口 (clever, sensible) with the honorific prefix お and the suffix さん, creating a noun phrase used to praise children.