Not sure if this would be the most cost effective solution but maybe some pc water cooling parts, with clear tubing it would also allow them to see it flowing through the system. With quick release systems they might (age dependent) be able to re-arrange the system to experiment with different systems and flows.
Difficulty would be keeping it safe with regards to electric.
I bet you could rig a water cooling pump to run off a 12v power tool battery, and completely remove the need for dangerous wall AC power from the equation.
Difficulty would be keeping it safe with regards to electric.