childishness       
noun [ U ]  disapproving uk/ˈtʃaɪl.dɪʃ.nəs/ us/ˈtʃaɪl.dɪʃ.nəs/
 (成人)孩子气,幼稚
the quality of being something that would be expected of or appropriate for a child
  Ashamed at my childishness, I apologized. 我为我的幼稚感到羞愧,我道歉了。 
 Complaining to your mother like this is pure childishness. 像这样向你母亲抱怨是纯粹的孩子气。 
 See: childish
 - More examples
 - To most people, her occasional childishness seems merely charming.
 - I was behaving with the childishness of a newly published writer and complaining about my book.
 - If there's one thing that defines their work to date, it would have to be childishness.