Possible Clues:

• One's youthful side
• Grown-up's playful side
• What might make an adult swing on a swing set
• Part of a person's psyche ... or a hidden part of 18-, 23-, 39- or 48-Across?
• What each of the entries with circles reveals
• What might make an adult jump in a pile of leaves
• Psychology subject ... and what's contained in this puzzle's circles
• Focus of a 1990 psychology best-seller