If you want to be a goody two-shoes, buy the game, download the .exe, and "lend" it to him - never play your GOG version while he’s playing, pretend it's a physical media you have only one copy, and just keep playing your Steam version.
However, if you read their terms, it’s actually legal for you to keep two copies of the game - like one on the device you’re playing and another somewhere else as a backup. So you can legally lend your game and still keep a backup copy.

Iirc, GOG staffers consider ok to let people from a same household to use an user's games. But you can confirm that in their official channels, and as you already have an account on GOG by the way you wrote, you can post and comment in their forums too, where staffers often lurk.

Well, if you have any interest in the game yourself I’d say just make yourself a GOG account, buy the game, and put it on a cheap flash drive and give it to them. GOG doesn’t have DRM. I’m not saying cheat or steal, but no DRM means they’re giving you more agency in what you can do with the game. That is a valid reason to buy from them. Technically, as long as you’re not both playing at the same time, legally it’s the equivalent of sharing a physical Nintendo game.

Terraria is on sale and i thought i'd buy a copy for my kid brother. But then i thought i should actually read the user agreement and privacy policy before i tell him to make an account. Turns out people younger than 16 can't own accounts, and i can't just buy a copy for him on my account because i'm already going to buy a copy for my account (i currently have the Steam version but want the game sans DRM).

So if i want to get him this game, should i make an account in his name but keep control of it until he's older? Is there some better legal option here?