The braitta (Sotah 11b) is suggesting that they provided for them even beyond their initial infancy, to the point where they could be self-sufficient (i.e. "וַתְּחַיֶּיןָ" suggests that "they enabled the boys to live").
Possibly, at that stage of the anti-Hebrew decrees, Pharaoh was only requiring feticide or partial-birth abortion, not outright infanticide (Exodus 1:16: 'And he said, "When you deliver the Hebrew women, and you see on the birthstool, if it is a son, you shall put him to death, but if it is a daughter, she may live."'). So once the infants were fully born, the mothers would be able to nurse them. The chiddush of the braitta is that they provided for them even beyond that point, when their care might have otherwise been prohibitively expensive for the Hebrew slaves.
Rashi ad loc (hattips @Danno and @IsaacMoses):
ותחיין - הוה ליה למיכתב ולא המיתו הילדים. ותחיין משמע שהיו מסייעין להחיותן שהיו טומנות אותן בבתיהן ומגדלות אותם:
and they enabled the boys to live - He should have written "and they did not kill the children". "and they enabled the boys to live" implies that they helped keep them alive - that they hid them in their houses and raised them.
(A counterpoint: The fact that Rashi says "they hid them" suggests that this was also illegal at least by then. If that is the case, there's still no reason to assume that they could not have smuggled in the infants' mothers to nurse them or provided a wet-nurse by other means. The chiddush of the braissa is that they indeed went even beyond that.)