From the Super Mario Wiki
Baby Cheeps are small Cheep Cheeps that Big Berthas take care of in Super Mario Bros. 3. Big Berthas occasionally spit them out and swallow them. In case Mario (or Luigi) defeats the baby (with a fireball, for example), Big Bertha will continue to "swallow" the area the baby used to be, and will spit out a new one later, as if the original had never been defeated.
Three Baby Cheeps appear in the The Adventures of Super Mario Bros. 3 episode "Reptiles in the Rose Garden". A single Baby Cheep appears in the Nintendo Comics System comic Love Flounders as the child of The Big Bertha who falls in love with Mario.