Buying the game now requires navigating a couple sites, back and forth, in the correct order, and getting past MS attempts to block seemingly any new account for non-existent "suspicious activity". If you're buying for a kid and make the mistake of not lying about their age, you'll also experience the hell of MS' family account management interface, including having to track down an obscure and not-obviously-related setting to let the new copy connect to any multiplayer server, including local ones. There are, of course, multiple game-related settings screens, because why would it make sense? And only one of them has what you need. Plus you need to visit it in the correct fashion to have it apply to the child account, or else it won't work.

And you'll need to juggle logins to both accounts—the parent account, and the child account—and bounce between them a couple times to get it all working. There's no way normal users are managing to do it successfully.

As for the account transitions, it took me a couple tries to get mine working, and my wife's tried several times and they keep telling her on her MS account(s) that she doesn't own Minecraft and needs to buy it. I haven't looked into it, but I imagine she's missing some non-obvious step. Her experience is likely pretty common.

[EDIT] Oh, another thing I haven't looked into yet: as of a few days ago its started telling my kid they don't own it, and we need to buy it. They fucking definitely do own a copy. No idea what's up with that, and I'm dreading having to figure it out.