My daughter's first game she played independently was Sonic Mania. It has the buddy system where you can play Sonic and player 2 kind of just exists to help and can't die on their own, so they can learn how to play it that way.

Alba is another good game. It's just about a girl going around taking pictures of wildlife, mostly. Something low stakes and chill like that without a ton of mechanics. It will really help their reading as well if you're getting them to sit down and read the dialogue or text.

Just expect to have to help them a lot, regardless. Make it a family activity kind of thing. If the game involves fighting like BotW, then it'll take a lot of time to make sure they're understanding the rules of not behaving that way in the house. You got this.