Reading manga is usually seen as something that entertains children.
Literal
Manga [object-を] read thing [topic-は] usually children's entertainment [quotative-と] is-seen.
The passive 見られる ('is seen, is viewed as') combined with と (quotative) forms the standard 'is regarded as X' pattern. 気晴らし ('entertainment, diversion') is built from 気 ('mood') + 晴らす ('to clear'), literally 'clearing one's mood.' こと nominalizes the verb phrase 漫画を読む ('reading manga') to serve as sentence topic.