Puerile

[Sarah]

Boyish; childish; trifling; silly.The French have been notorious through generations for their puerileaffectation of Roman forms, models, and historic precedents. DeQuincey.

[Sarah]

Childish; suited for children, trivial

[Renato]

Act of [childish] [behaviour]

[GJB]

[Rihanna]

behaving in a silly way, not like an adult: