The song I am referring to is 봄 by 윤석중, a well-known writer of Korean children's poetry. This poem was later composed into a song in 1990.
니나니 나니나 니나니나 버들피리 소리가 들려온다
니나니 나니나 니나니나 버들피리 소리가 들려온다
니나니 나니나 니나니나 시내에 얼음이 다풀렸다
니나니 나니나 니나니나 잔디가 파랗게 돋아난다
니나니 나니나 니나니나 산나물 캐러들 올라간다
니나니 나니나 니나니나 제비가 물차고 날아든다
I am having some difficulties understanding the last two lines. In particular, the meaning of "캐러들" and "물차고". As for "캐러들" I really have no idea except it might somehow be related to the verb "캐다" (to dig out). Whereas for "물차고", it can mean "to be filled with water" but it certainly does not make sense here (like, how can you fill a swallow with water?)