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If children are sinners, why did Jesus say we must become like them to enter into the kingdom of God?

It isn't because of any supposed "innocence" on the part of the children! It is because children are dependent and humble, willing to accept from another what they cannot provide for themselves. And what is it that we sinners cannot provide for ourselves? Forgiveness, deliverance from the power of sin over us, from the world, the flesh, and the devil.

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Adam and Eve were created with the God given freedom to choose to sin or to choose to trust and obey Him. ... In the same way every child is born with the freedom to choose to sin or to choose not to, and is sure to choose to sin if they live long enough.

Adam and Eve, before the Fall, were clearly capable of living out either choice - either innocence or evil. Once they yielded to sin, however, they and all their descendants became enslaved to sin (Romans 7:14-8:2). Free will is not possible for slaves. No matter what they wish, they are not able to live out their own will as long as they are prevented from doing so by their chains. They must first be freed! That is the sense of the doctrine of Total Depravity that we chafe against.