Yeah. I think age transformations including babies is probably the rarest. One of the very reasons why this fetish is not as popular as others is because it makes some people feel uncomfortable as it usually involving underage girls. So stuffs featuring infants can come off as especially sensitive to the public and are therefore pretty rare among media. When they happen in anime, the theme would usually have to be somewhat light-hearted, narrative, artistic, comedy, and/or fantasy, just like those episodes of Nana Seven of Seven and Classicaloid.
Among my favorite transformations, the one I found most interesting would be from the old anime movie "Paprika", where the female protagonist becomes a giant baby that sucks in all the nightmares and grows into an adult and brings the world back to normal. I found it interesting because it introduced me to the combination of absorption and AP, where the character takes in certain amount of mass into her belly, and that mass visibly expands and converts her body towards maturity.
https://www.youtube.com/watch?v=Z7IKd7p59do
As for my other favorites I can immediately think of, they would be the 3D animations from apfan101 whom I created a thread about some time ago.