Not on a bike, but when I was 5 a friend and I were scrambling on the "cliffs" in a neighbor's front yard when I slipped backwards and fell about 5', landing forehead-first on a sloping rock. Wound was small but bled "like the Red Sea" (friend's description). No obvious long-term effects, though others might disagree.
On a sadder biking note, about 5 years ago I reconnected thru FB with my best friend (ages 8-22) after nearly 50 years. He moved to Idaho shortly after finishing college and at reconnect time he was dominating the state's age-class mountain bike races. A couple months after the reconnect he took a bad fall at speed while practicing, resulting in serious brain damage. Then cascading issues culminating with a spinal cord infection took his life.