Psa 51:5 Behold, I was shapen in iniquity; and in sin did my mother conceive me.
We are born with a sin nature and this can be plainly demonstrated. I have yet to meet a baby or young child that needs to be taught to do wrong but they all need instruction in what is good and right.
Sweet yes, born sinless no. Until they reach the age of accountability they are certainly Christ's.
Scripture indicates that when man partook of the tree of the knowledge of good and evil that he could no longer stand in God's presence. Man was once led of God but now he was dependent on his own mind for direction- the carnal mind is always at enmity with God and even man's "good" is but a filthy rag.
These are just my views and I do of course respect your right to your own.
There is a teaching I heard on the radio many years ago while living in the San Francisco Bay Area from the radio preacher Campbell. He was saying that, if I remember correctly, children who die before reaching accountability are not in a state of innocence and may go on to eternal punishment. I am not really up on my Calvinist theology but that probably goes hand-in-hand with a Calvinist belief with its predestination.
Calvinism is EXTREMELY hard to discuss with all of its ins and outs so I would not want to try and address it here. I like to keep things simple. One scripture that is commonly used to address the subject of innocence is where David was greatly lamenting the sickness of his son born from Bathsheba. When the son died David got over it in an instant explaining that he would see his son again. I am therefore more inclined to believe that children have a state of innocence and go on to be with the Lord. It is God's perogative to determine when they no longer have innocence.
With Mark 10:4 I would hesitate to draw on it as a salvation doctrine. To do that there would have to be something else in the passage suggesting a release of iniquity message. The one clear message we can draw from it is that children should be an example to us of how to act within the kingdom of God with all of their joy, humility, and exhuberance. Please keep in mind that the kingdom of God is with us here and now. As Christians we are part of the kingdom of God and live it here and now. It is not to be confused with the millenial reign of Christ or the eternal kingdom.