Dr. R.C. Sproul often notes the difference between childishness and childlikeness. Believers must be childlike in that they trust and believe God without hesitation, just like little kids trust their parents. However, Christians cannot be childish, never having anything more than an elementary knowledge of the faith. Young and old alike must be growing in their knowledge of God, trusting Him like a child while maturing in their doctrinal comprehension.
For further study:
The Bible in a year: