That is, just get a smallish chassis box and mount a couple
of standard jacks on it. Then leave open one side of the
box so you can complete the circuit using alligator clips.
Soldering a small loop of wire onto the jack terminals makes
them easier to grab.
In the picture the circuit is already completed by resistors.
You would want to omit the resistors.
Such a box is cheap and easy to make, and gets re-used a lot.
Also it sets a good example for the kids, showing that you
can flip the phase (and insert voltmeters, and do a lot of
other things) without having to cut the wires.