immature, childish

(ようち) Childish, immature, puerile. “Hey I wrote POOP on a picture of the president! Want to come see?”
 
(おさない) Like 幼稚, but one difference:if you use it about an actual child, it doesn't have a bad meaning. It just means ‘of children’ – like  ‘childhood memory’ or ‘At an early age,’ or ‘in infancy.’