When did it become regulation to list power requirements on devices? (I'm wondering if this toy pre-dates that time or not). Usually if you check the back or bottom of a device like this, it will list power input ratings for the jack (assuming there's a jack).
Next thing is, check the OTHER side of the pcbs. They could be blank, or they could be marked with more information. (For example on this visible side, we see markings like "C1" which means Capacitor. "R1" means resistor, etc. There might be a voltage indication somewhere).