This is a difficult problem. After all, readers would not pick up biographies of random children; it is the ultimate role that a figure plays that make him or her the proper subject for a life study. We want to know the roots of later life, to see how the more famous (or notorious) aspects of a life germinated in youth.
All too often, however, the biographer falls into a kind of crude determinism. The path appears clear from the beginning; everything seems destined; the talents of a full-grown adult appear clearly in the youth. In reality, of course, a life plays out in an unplanned chain of accident and intention, of unexpected opportunities and unwanted challenges.
As with everything, capturing that sense of contingency is a delicate matter. Overdone, it drowns the reader in detail, erasing a sense of forward momentum that we wish for in a narrative. Underdone, it makes the child seem less like a child than a small adult—or an actor standing in the wings, waiting for the cue to enter the stage and play a well-rehearsed role. Dramatic irony is often useful. A reader of a biography likely knows something of the subject's ultimate role; keeping it in mind, the biographer can surprise and create narrative tension by showing how the subject nearly went in this or that direction. If the subject rises to great accomplishment, then early failures and disappointments should not be downplayed; they should be told fully, to add to the dramatic impact of the ultimate achievement.
It's hard to capture immaturity, childishness, undeveloped sensibilities. Sometimes a biography doesn't reach so far back, and these are irrelevant. But the worst thing is to pretend that they didn't exist—that the subject was mature and accomplished from the start. There's no better way to flatline your narrative arc.