Another idea for a breakaway system that I saw:

1) Burn the ends of (synthetic) cord to make them hard
2) Take a short (1 inch) piece of surgical tubing with a diameter just thinner than that of the cord.
3) insert each end of cord into each side of the tube to join the lanyard
4) test for the right tension, and fiddle to fine tune it

I havn't tested this method in the field, but it seems an effective idea.

For children though, why not try get hold of "dog tags" that they use in the army. they have identification tabs, a built in breakaway system (I think), and they have the whole """cool"" army/military/tough guy look.

my 2c