People were bringing children to him that he might touch them,
but the disciples rebuked them. 
When Jesus saw this he became indignant and said to them,
“Let the children come to me;
do not prevent them, for the kingdom of God belongs to
such as these.
Amen, I say to you,
whoever does not accept the kingdom of God like a child
will not enter it.”
Then he embraced them and blessed them,
placing his hands on them.  Mark 10:13-16

it is difficult to be like a child in the matter of the Kingdom of God

why?