I think you have the answer, you own two accounts, let child use one. Gift account to child on their 16th birthday.
GOG
GOG.com is a DRM-free games and movies distribution service that is part of the CD Projekt Group. GOG.com is also a "sister" company to CD Projekt Red, developers of the Witcher series and Cyberpunk 2077.
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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.
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.