"You know how children are."
Translation:Você sabe como são as crianças.
I was always under the impression that it was okay to omit the pronoun when used with a verb, as the ending tells you which person it is. Like "Preciso de pão", where 'eu' may be omitted. But "Sabes como são as crianças" was marked incorrect. Is this not the case when using "tu"?