Could we have a parent account for our child so that the games that are rated M/AO for mature are not shown? He does have a steam account using their system to restrict content (MA/AO/PG) and it is restricted to games that are outside his rating and will not be shown in his client (nor advertisements.

As an example Worms Revolution is a game that would be a fun game to play with him. I do not know how must cross platform (steam, gog galaxy, epic digitial store) work with mutiplayer.

Thank You!

Feel free to vote for the relevant entries in the community wishlist:
• Allow child accounts to be added to parent account
• Parental Controls

You can also submit your feedback via support ticket. As you can see from the bottom of the game's description, the GOG version only supports local multiplayer (i.e. not over the Internet). If that's a feature you're interested in, you have to research it for each game beforehand.

how it works in general is one becomes the host and all others join that host... in the old days that was two players that share their ip address but most games now use a server host normally rented or run by the company that made the game
so for example Axis & Allies is run by Beamdog and BG3 is Larian and players with Steam or Gog can play against each other because the server is set up correct to allow the ports needed