The old Daisey lever guns shot a BB at around 450 feet per second. With the modern pump-up pellet and BB rifles (like the 760, IIRC) the BB can reach 800 or 900 feet per second. With a four year old, the bone plates int he skull have yet to harden and this might allow a BB from even an old lever rifle to penetrate. But if a ‘hi-powered’ gun was found it was likely one of the pump up jobs which can be pumped ten times for a shot. There are also single lever (the barrel is the lever, increasing leverage) rifles which can exceed 1000 feet per second (google ‘GAMO’).
Daisy lists the Red Ryder at 240 FPS. Test have shown it can deliver up to 375 FPS at less than 10 yards. After that, velocity and accuracy degrade considerably.