This is a costly solution and requires a lot of hardware...but it is a sure thing!!!
If you cannot do it through software...let's look at the hardware options then.
The best thing that requires no (or hardly any) supervision from you is to cut and enable the power to the PC's from your kids between certain hours. Works like a charm and there is nothing they can do to a lack of power...especially when the timer is securely locked away. Each PC should have a small UPS, so they have some time to save their homework though.
Such a timer looks quite costly to me and I assume the electrical system requires some (heavy) altering...but hey, it's your piece of mind after all.