Let us trust as a child does.
When Jesus says to be like children, he's not in any way saying we should be childish. What Jesus is referring to is that we need to return to being like children in terms of their purity.
This means having the confidence of a child, having the humility to recognize that we depend on God, and also the simplicity of a person who doesn't seek complications, who seeks simplicity. A child generally doesn't worry about what will happen tomorrow, nor does he think about how he'll pay his debts or whether his father will take care of him or not. A child simply trusts, believes, and simply entrusts himself to his father's care.
And this is exactly what Jesus seeks from us. He seeks for us to put aside pride, to leave aside excuses or self-sufficiency, and to simply return to depending solely on him with a simple, humble heart, trusting in his promises.
And he said, "Assuredly, I say to you, unless you change and become like little children, you will never enter the kingdom of heaven."
Matthew 18:3