Do what I did. After 20 minutes, turn the cold supply to the water heater off. Do this a few times.
When they ask why the hot water ran out, act dumb and say you’ll have to check on it.
After a few more times, do that after 15 miutes.
Doesn’t take very many cold end of showers with shampoo in their hair to make sure they get done before they run out of hot water.
After a few times, you won’t need to do it any more. Then the trouble and cost to install the solenoids and timers would have been wasted.
Be nice to your daughters though, make sure they get plenty of warning the long showers will cause the hot water to run out. Then your predictions are only coming true when the hot water actually does run out.
(Just don’t forget to turn the cold supply valve back on.)