I've built something similar to this, but I don't think mine has anything to do with actual clock signals. I think the important thing is to find bend points in the circuit that will dramatically alter the pitch when connected with little resistance. If you are there already, then my circuit should work for you.
Like mikebike was saying, I made mine with a pot connected to each switch to allow for tuning, rather than hard-wiring the resistances. I think you will find tuning with resistors difficult to do, especially if you are planning on using this across multiple toys. In my case, the pitch of the signal the keyboard effects is not affected in a linear way. Being able to tune on the fly is quick and easy.
The only thing I don't like about my circuit is that it plays a tone when no key is pressed. I think that is what Colin was getting at, as completely cutting the connection would crash the circuit. However, if you can get the pitch above audible frequency, you could make your circuit default to that.
The schematic is essentially the same as what you drew, but with pots in place of resistors. I also added a LDR with a switch for alternate control.
As for connecting with alligator clips, if you are looking for a cleaner alternative, a simple audio cable with jacks attached to bend points works great.
Check it out here: http://www.vimeo.com/4230827