Block internet access?

I previously set up a Raspberry Pi (using Raspbian) as a Wireless Access Point to create simple controlled internet access for my kids.
It connected to my main router over ethernet (eth0) and provided a separate wifi SSID on a separate subnet for the kids.
I used some simple scripts to change /proc/sys/net/ipv4/ip_forward to allow routing between wlan0 and eth0 to control internet access. This meant I could setup some simple schedules using cron to run the scripts. I could also run the scripts manually over ssh to override the cronjob schedules to provide time extensions, or to block the internet immediately.

I have now bought a GL-iNet AR300M-Ext Mini wifi router running OpenWRT/Lede to replace my Raspberry Pi to provide some extra performance and flexibility.
I started with the smae scripts I used before, but ip_forward does not seem to be settable - I cannot set it to 0 to block internet access. Why is this? What is preventing it?

After browsing this forum, it seems the preferred way to do this is now using firewall rules according to Parental Controls
But how can I override these rules to temporarily turn internet access on/off when I desire? Like big red and green buttons to turn internet access immediately off and on again.?

You can enable/disable specific firewall rules either from LuCI or via SSH-command.

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Thanks. That's good to know.
I'm not too familiar with OpenWRT/Lede's configuration with (L)uCI yet or its firewall.
You make it sound like I can easily add or remove a single firewall rule, rather than having to clear all the rules and write a different set of rules, is that right? (I guess I must still restart the firewall for the rule change to take effect?)
Can you provide a link to an example that does this sort of thing as a starting point for me?

https://openwrt.org/docs/guide-user/start#firewall_configuration