The reason for this is not because these children are more humble, more trusting, or more innocent than adults or other children. The kingdom of God belongs to covenant children simply because Jesus has granted covenant children the grace to belong to his kingdom. In Matthew 11:25–27, Jesus praises his Father that he has chosen to hide the kingdom from the wise and reveal the kingdom to little children.
The kingdom of God belongs to covenant children not because they have taken steps toward God, not because they have the seeds of faith within them, not because they excel in humility and innocence. It belongs to them because God has determined that his promises are also for them (Acts 2:39).
14 But when Jesus saw it, he was indignant and said to them, “Let the children come to me; do not hinder them, for to such belongs the kingdom of God.