Ben,
Common methods have included putting a 1 ohm, 5 watt resistor (or a 2 ohm, 10 watt) in series with the voltage to the center rail or a series of bridge rectifiers that would reduce the voltage. In both cases, the components will get warm/hot and should be kept away from little hands. These methods have been discussed in some other threads. Here is one example (see drummer3, Dennis LaGrua posts):
https://ogrforum.ogaugerr.com/...70#77734316074624270
There are other possibilities but more complex like using zener diodes and SCR's, but the above suggestions should suffice.