By "dsn" I assume you mean DNS. DNS can be changed at the adapter level on the laptop, so if it's wifi or an ethernet connection you could change it to OpenDNS IP addresses there on the adapter itself: E.G.: https://www.windowscentral.com/how-change-your-pcs-dns-settings-windows-10
Make sure you do it for both the LAN and Wireless adapters. I also have AT&T Fiber, the only way to assign OpenDNS on your network is to do the above or buy a separate wireless router, which could be a pain to setup if you're not that technical.
DNS assigning on the adapter is very easy to do though, I would go with that. It would only effect that one device though.
Use these two IP's:
Also, if you do this, make sure you create Non-Administrator accounts for the kids so they cannot change this setting.
Regarding blocking ads and things like that I use the Extension "Ublock Origin", it's free and works great on Firefox. It even removed ads from YouTube.
You could also install the Convenant Eyes software on the laptop, which generates reports for you if they view anything dicey. It's 12$ a month, but can be used on multiple machines, and smartphones.
I hope that helps. I've been working IT for 15 years, so I'm pretty familiar with how to lock-down PC's. If you have any more questions let me know.