Do you think He might've been referring to their helplessness?
After all, He was rebuking apostles for exclusionist behavior.
The association with of the kingdom with children was not in the context of a discussion on original sin:
Mark 10:13-16 13 And they brought young children to him, that he should touch them: and his disciples rebuked those that brought them. 14 But when Jesus saw it, he was much displeased, and said unto them, Suffer the little children to come unto me, and forbid them not: for of such is the kingdom of God. 15 Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein. 16 And he took them up in his arms, put his hands upon them, and blessed them. The remark was in reference to their vulnerability, it had no positive inference about their character. Jesus was rebuking apostles for exclusivist behavior.