Matthew 18:1-6 is a key passage:
18:1 At that time the disciples came to Jesus, saying, “Who is the greatest in the kingdom of heaven?” 2 And calling to him a child, he put him in the midst of them 3 and said, “Truly, I say to you, unless you turn and become like children, you will never enter the kingdom of heaven. 4 Whoever humbles himself like this child is the greatest in the kingdom of heaven.
5 “Whoever receives one such child in my name receives me, 6 but whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened around his neck and to be drowned in the depth of the sea. (All scripture quotations from the ESV)
Childlikeness, not childishness, is God's ideal for our behavior:
1 Corinthians 13:11 When I was a child, I spoke like a child, I thought like a child, I reasoned like a child. When I became a man, I gave up childish ways.
and Ephesians 4:11-16
11 And he gave the apostles, the prophets, the evangelists, the shepherds and teachers, 12 to equip the saints for the work of ministry, for building up the body of Christ, 13 until we all attain to the unity of the faith and of the knowledge of the Son of God, to mature manhood, to the measure of the stature of the fullness of Christ, 14 so that we may no longer be children, tossed to and fro by the waves and carried about by every wind of doctrine, by human cunning, by craftiness in deceitful schemes. 15 Rather, speaking the truth in love, we are to grow up in every way into him who is the head, into Christ, 16 from whom the whole body, joined and held together by every joint with which it is equipped, when each part is working properly, makes the body grow so that it builds itself up in love.
In fact, we are expected to become more mature:
Hebrews 5:12 For though by this time you ought to be teachers, you need someone to teach you again the basic principles of the oracles of God. You need milk, not solid food, 13 for everyone who lives on milk is unskilled in the word of righteousness, since he is a child. 14 But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
Children, not just a childlike attitude, are important to Christ:
Mark 10:13 And they were bringing children to him that he might touch them, and the disciples rebuked them. 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. 15 Truly, I say to you, whoever does not receive the kingdom of God like a child shall not enter it.” 16 And he took them in his arms and blessed them, laying his hands on them.
See also the first quotation above, which shows their importance.
Thanks for reading! Have a childlike faith, but grow up.