You could install a small tankless unit just for the shower they use. This would free up the tank volume for use in the rest of the house.
Even better, install a larger tankless unit to supply the whole house, then you won't run out anywhere.
I'd be glad to help you size the heater. Do have a gas or electric water heater? Both are available in tankless (gas units produce more hot water volume).
Screaming at the kids, and running out of hot water, are the two main reasons folk buy tankless. Just kidding about the screaming part, but running out of hot water is very frustrating :O(
I can just imagine you at shower-time "I'm going to the basement now" clunk, shower shuts off :)
One of the guys is right on the money; In England, many small hotels & B&B's have coin-operated timer-type tankless shower heaters. I sell those too :) but you might just wind up with an extra nickname or two, cheapskate comes to mind.