Jesus was using young children both to make a point about how adults needed to become "as little children" to enter Heaven AND to impress on His audience that mistreating children was a horrible sin. Consider these similar verses from Mark:

Mark: 
10:13: And they brought young children to him, that he should touch them: and his disciples rebuked those that brought them.


10:14: 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. 


10:15: Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.

Jesus was angry with his own disciples because they disrespected some children. He also stated plainly that anyone who is NOT "as a little child" isn't getting to Heaven -- Heaven and "the Kingdom of God" having the same meaning.

Verse 10:15 is ambiguous (at least as translated into English). I believe Jesus meant BOTH that:

• A newborn child IS ALREADY in the Kingdom, but abusing a child may damage that person to the point where they are separated from God, and

• A damaged adult may FIND THEIR WAY BACK TO THE KINGDOM by recovering their own innocence and purity of heart -- by recovering the characteristics of a healthy young child, which are ALSO characteristic of a healthy ADULT -- of which, there have always been too few.

That interpretation fits well with Luke 17:21, another direct quotation from Jesus:

Luke: 
17:21 Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you.