Become as Little Children

"At the same time came the disciples unto Jesus, saying, Who is the greatest in the kingdom of heaven?
And Jesus called a little child unto him, and set him in the midst of them,
And said, Verily I say unto you, Except ye be converted, and become
as little children, ye shall not enter into the kingdom of heaven.
Whosoever therefore shall humble himself as this little child, the same is greatest in the kingdom of heaven.
And whoso shall receive one such little child in my name receiveth me.
But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.
Woe unto the world because of offences! for it must needs be that offences come; but woe to that man by whom the offence cometh!
Wherefore if thy hand or thy foot offend thee, cut them off, and cast them from thee: it is better for thee to enter into life halt or maimed, rather than having two hands or two feet to be cast into everlasting fire.
And if thine eye offend thee, pluck it out, and cast it from thee: it is better for thee to enter into life with one eye, rather than having two eyes to be cast into hell fire.
Take heed that ye despise not one of these little ones; for I say unto you, That in heaven their angels do always behold the face of my Father which is in heaven."
-- Matthew 18:1-10
Another saying referring to small children can be found in the non-canonical Gospel of Thomas:
"Jesus saw some babies nursing. He said to his disciples, "These nursing babies are like those who enter the (Father's) kingdom". They said to him, "Then shall we enter the (Father's) kingdom as babies?" Jesus said to them, "When you make the two into one, and when you make the inner like the outer and the outer like the inner, and the upper like the lower, and when you make male and female into a single one, so that the male will not be male nor the female be female, when you make eyes in place of an eye, a hand in place of a hand, a foot in place of a foot, an image in place of an image, then you will enter [the kingdom].
Jesus said, "I shall choose you, one from a thousand and two from ten thousand, and they will stand as a single one.
His disciples said, "Show us the place where you are, for we must seek it."
He said to them, "Anyone here with two ears had better listen! There is light within a person of light, and it shines on the whole world. If it does not shine, it is dark."
Jesus said, "Love your friends like your own soul, protect them like the pupil of your eye."
Art -- Christ with children by Carl Heinrich Bloch