This is the 5503rd most frequent Korean word.
머금은
"Containing" or "holding (in the mouth or heart)."
In this sentence, '머금은' is used to indicate that the face possesses or is holding the warm smile, metaphorically as an expression.
그녀는 따뜻한 웃음을 머금은 얼굴로 나를 바라보았다.
She looked at me with a face full of a warm smile.
Here, '머금은' conveys the flower containing or retaining dew physically.
작은 꽃이 이슬을 머금은 채 아침 햇살을 받았다.
The small flower held dew as it received the morning sunlight.
In this instance, '머금은' describes the figurative context of the child's eyes showing curiosity.