Informal:
* Babble: This implies nonsensical or incoherent talk.
* Whimpering: This suggests complaining or whining.
* Pettiness: This implies trivial or insignificant talk.
* Nonsensical chatter: This suggests meaningless or silly talk.
* Immature talk: This is a straightforward way to describe talk that is inappropriate for an adult.
Formal:
* Juvenile discourse: This is a more formal way to describe talk that is childish.
* Frivolous conversation: This suggests talk that is lighthearted and unimportant.
* Superficial remarks: This implies talk that lacks depth or substance.
* Naive pronouncements: This suggests talk that is uninformed or inexperienced.
The best word to use will depend on the context and the specific type of childish talk you are trying to describe.