Yeah I'm not sure about mounting the Wii to a wall.. But you can always lock it into a clear plastic case, and leave the key at the front desk..
You probably need an employee to change the games every now and then instead of leaving the games out. Kids can go to the front desk and ask for a different game while the employee would be able to grab the key and swap it out.
You wouldn't want it just mounted to wall, unprotected, because it would be too easy to steal the game that's already inside the machine.