It's FOSS published a tutorial about how to limit computer usage per account in Linux with Timekpr-nExt.

Got Kids? Limit Computer Usage Per Account in Linux With Timekpr-nExt

Timekpr-nExt lets you limit the computer usage on a Linux system. A handy utility for parents who do not want children to spend excessive time on the computer.

Got Kids? Limit Computer Usage Per Account in Linux With Timekpr-nExt - It's FOSS