Restrict computer time for kids with crontab
sudo apt-get install -y git
git clone https://github.com/VeggieVampire/bedtime_crontab
cd bedtime_crontab
chmod 777 *
mv bedtime.sh ~/bedtime.sh
mv crontab ~/crontab.new
cd ~
crontab crontab.new
Now edit bedtime.sh and change(programz=chromium) to whatever program you might want to keep from being used.
programz=chromium