"People were bringing little children to Jesus to have him touch them, but the disciples rebuked them. When Jesus saw this, he was indignant. He said to them, 'Let the little children come to me, and do not hinder them, for the kingdom of God belongs to such as these. I tell you the truth, anyone who will not receive the kingdom of God like a little child will never enter it.' And he took the children in his arms, put his hands on them and blessed them."- Mark 10:13-16Every time that I can recall that I have heard this taught it has been about the faith of a child. The lesson is on how children don't question things and just accept them, so we should accept God in the same way. While faith is important, I feel that the more important and challenging lesson has been dropped from this telling of the passage, which is humility.
There are two hints to what the actual point of the passage is. The first hint is that the disciples find it necessary to rebuke people who are bringing children to Jesus. Why do the disciples feel the need to specifically keep children away from Jesus? It doesn't have anything to do with the children's faith. That would make no sense.
The reason that the disciples would have rebuked those bringing children to Jesus is that in Jesus' time in the Middle East there was no honor in being young. Honor was for the aged. In this patriarchal society the dishonorable youths were managed by the women, and for a male teacher to be wasting his time with kids when he could be teaching those who were older would have been a dishonor. That was a woman's work! Therefore, the disciples believe they are protecting the honor of their teacher. Furthermore, when Jesus tells the disciples that they will have to receive the kingdom of God like children he is telling them that they have to dishonor themselves to become a part of this kingdom.
The second hint is a bit more straightforward, but you have to look in a different passage to find it. Not long before this exchange in Mark 10 (also in Matt 19:13-15 and Luke 18:15-17) Jesus had already told the disciples that they would have to humble themselves like children. Notice this in the passage below.
"At that time the disciples came to Jesus and asked, 'Who is the greatest in the kingdom of Heaven?' He called a little child and had him stand among them. And he said: 'I tell you the truth, unless you change and become like little children, you will never enter the kingdom of Heaven. Therefore, whoever humbles himself like this child is the greatest in the kingdom of Heaven.'"- Matt 18:1-4It is good to have the faith of a child if it is truly faith and not a show, but in the passage where that phrase originated God is instead asking for your humility.