He was aware that he was still a child
Is consciente the best way to say aware? Era consciente de que todavía era un niño.
Yes, but I might use Estaba instead or Era just because the sentence seems to indicate a moment in time as opposed to something he always knew. "Estaba conciente de que todavía era un niño."
You could also use the phrase darse cuenta de, like "Se dio cuenta de que todavía era un niño." But that has the feeling more like "He realized he was still a kid."