Just because one option is better than another, doesn't mean it's good.
OS level age check applies to everyone, not just children. Some legislations require strong age checking, which means you need to send some identification to some service. You won't be able to know how the information is handled, for how long it's stored and for what purposes it's used beside age checking. And because this applies to everyone, and is required to be able to use your computer, everything you do with your computer and phone is tied to your user account, and as such to you as an individual and identifiable human being.
Some of these legislations uses age ranges, and the OS is required to inform applications, and such, whether the user is, for example, below 13 years old, or 13 to 16 years old, etc. Consider this simple scenario: Some user uses some application, and the OS reports the user's age as below 13. The user uses the same app the next day, but now the OS reports the user's age as 13 to 16 years old. Can you figure out the user's exact birthday and age? If that application is part of some kind of larger network of advertisers and whatnots, they will now forever know the user's exact age without the OS reporting anything else.
These can also be used to make some software illegal, especially free and open source software. If you can replace Windows with Linux, Photoshop with Gimp, etc. it hurts the bottom line of those companies. Those companies can't prevent you from using the open source alternative, but it would be in their interest if those pieces of software becomes illegal to use and distribute. If age checking functionality is added to some open source software, the age checking can simply be removed by the user. You only need to correctly form the age checking law and that entire software is now illegal, and must be removed from the internet.
While the intention of these laws might to be to protect children, they cause too much harm for little good. The age checking can be circumvented in some situations, meaning the children aren't protected. And the entire thing is a huge privacy mess (data leaks, etc.) for every single computer user.
If running Home Assistant is all you are going to do, Pi is enough. There's also official hardware with Home Assistant preinstalled: Home Assistant Green