Here are examples of the word/phrase "childishness" used in a sentence and paraphrased in a sentence...
At some point their childishness will need to come to an end.
At some point their infantility will need to come to an end.
Thank you for putting up with my childishness when I was younger.
Thank you for putting up with my juvenileness when I was younger.
As those adolescents grew and matured, they lost their childishness.
As those adolescents grew and matured, they lost their infantility.
Other Examples...