Choose a suitable toy – one with a simple on/off circuit.
Cut the disk to fit inside the battery case.
Slip between the end of the battery and the battery compartment.
Screw the cover back on – make sure there's room for the lead.
Now just connect your switch and turn on the toy.
Now, whenever the switch is pressed, the toy will work.
Easy!
Next, look at extending reward times, turn taking with switches, and learning the importance of releasing a switch:
Toy Control Box
AbleNet Toy Control Timers