When using rich rules you can get really fine grain with what you want to do. If you have your childrens' devices on a seperate subnet, you could just block all hosts on that subnet with the following:
firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source address="192.168.10.0/32" drop'
Followed by a reload (firewall-cmd --reload). If you want this to be something that runs every evening for a set period of time you could throw it in a script and have a cron job run it at a certain time every night and then have the reverse done in the morning or whenever you specify.
If you want to block specific ports you can do so with the following:
firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source address="192.168.10.0/32" port port=443 protocol=tcp drop'
Ensure you always do a reload and your firewall rule is assigned to the appropriate zone and interface that your childrens' device traffic is coming in on. Firewalld was designed to alleviate the user unfriendly IP tables syntax and it does so quite well. For further information:
https://fedoraproject.org/wiki/Featu...rule_structure
I used the above link a ton when I was first learning how to use rich rules.