There is a another software solution that will do this.
AdGuardHome. Bonus is that it will adblock and allow you to give kid friendly DNS to their devices.
There are two routes to go with installing this. Either use the OpenWrt opkg version and the wiki entry to install it.
or use the manual thread which gives you the more up to date Edge build versions.
Once you have it installed and working you will then just have to define rules for your kids devices and set up which clients get what rules. The info for that is found here : https://github.com/AdguardTeam/AdGuardHome/wiki/Clients