This translation is not good.
Childish is an adjective: "infantil".
Criancice is a noun: an act that is childish, childish thing.
Yeah, I wrote "It is childishness" to keep it as a noun. You're right that the translation is a little slack, but I think the Portuguese still makes sense.
Let me present here a couple morphological resources of the Portuguese language.
- The ending
-iceis commonly used to construct derogatory nouns: criancice, burrice, chatice, mesmice, tolice etc.
- Similarly, the ending
-entois commonly used to construct derogatory adjectives: avarento, nojento, briguento, fedorento, birrento etc.
Those are not the only possible endings for derogatory nouns and adjectives, but are certainly the most used for new constructions of the kind.
Just a hint. ;)