I went through some of this myself.

> Nintendo Switch Online (not really another account, mind you, but a membership) involves a recurring fee. It also unlocks access to the Nintendo eShop, which I cannot disable. I can set his eShop spending limit to zero, sure. But I can't block free downloads. So to let my son play online Minecraft with his friends, I have to open him up to an unrelated store full of content I can't possibly evaluate. That's the deal. Take it or leave it.

You don't need to pay for Nintendo Switch Online to get access to the eShop, you just need a Nintendo Account. I made one for myself and one for my son, and neither stays logged in. My wife and I have the passwords for both, and will not give him his password until he's older. Meaning one of us needs to be there for any purchases, free or otherwise.

He has access to the Minecraft marketplace, but can't add funds to it without us. We did not use our MS accounts, and didn't make one for him, so he can't play Minecraft online. But you know what, I'm totally ok with that. He can still invite a friend over and play together in person (which he does do).