Midras Yoel 2:16: Mekhilta DeRabbi Yishmael & Midrash Tanchuma

R. Yossi says (Psalms 8:3) "From the mouths of olelim and yonkim You have founded strength. "olelim" — infants in their mothers' womb, as in (Iyyov 3:16) "Why was I not like a buried stillbirth, like olelim who never saw the light"? "yonkim" — those who seek their mothers' breasts, as in (Joel 2:16) "Gather olelim and suckers ("yonkei") of the breasts."

Rebbi says: "Olelim are infants outside (their mothers' womb), viz. (Jeremiah 9:20) "to cut off the olel outside," and (Eichah 4:4) "Olelim begged for bread. "Yonkim" are those at their mother's breasts, viz.: "and suckers ("yonkei") of the breasts." These and those (at the splitting of the sea) opened their mouths and chanted song before the L–rd.

R. Yosé the Galilean explained: Out of the mouths of babes and sucklings hast Thou founded strength (Ps. 8:3). Babes are those still in their mother’s wombs, as is said: Or as a hidden untimely birth I had not been; as babes that never saw life (Job 3:16). Sucklings are those who suckle at their mother’s breasts, as is said: Gather the children and those that suck the breasts (Joel 2:16). Rabbi maintained: Babes refers to children out on the street, as it is said: To cut off the children from the street (Jer. 9:20). Likewise it says: The young children ask bread (Lam. 4:4). All opened their mouths and sang the song unto the Lord. R. Meir declared: Even the embryos in their mother’s wombs sang the song, as it is said: Bless ye God in full assemblies, even the Lord, ye that are from the fountain of Israel (Ps. 68:27). Even the angels sang the song, as it is said: O Lord our Lord, how glorious is Thy name in all the earth! Whose majesty is rehearsed above the heavens (ibid. 8:2).