If aliens exist out in the universe somewhere on another planet, then so do alien babies. Since it seems that aliens have similar body types which are fragile, I would think that they reproduce in the same way that humans do. Therefore, they then produce alien babies and that these alien babies grow up slowly like human beings do and then become toddlers. Over time, alien babies grow up to become an adult alien.
The alien babies probably cry just like human babies do and they eat mashed up food and poop in their alien diapers. Once alien babies learn, they will become more independent. Then they become alien teenagers and then alien adults. At this point, the aliens have alien babies of their own.