My nearly 7 year old has a switch lite and loves it. There's a lot of choice for their age as even the big Mario games have an 'easy' mode so they can be played by everyone. I've got a switch too so we share some games.
Games he currently loves:
Super Mario Odyssey - This is your typical Mario game, v beautiful and you search through the world and collect power moons. Some boss battles and puzzles to solve/interesting missions. The game is massive so would take forever to properly complete/get all the moons. My son plays it on assist mode which means you don't lose your life that often and you get lots of tips/arrows on where to go. He's made good progress with it and doesn't get frustrated. One of my favourite games to play as well!
Minecraft - If he likes building stuff then he will like this. I don't massively understand it/what keeps him coming back to play it but it's the game most of his friends are playing and talk about all of the time.
Animal crossing - you have your own island and basically build it up, invite animal islanders to move in and plant flowers etc. You can spend your day fishing and collecting fossils to fill the museum. It's the perfect game to potter. There's no winning or losing and very relaxing!
Lego city undercover - he really loved this.
Yoshis crafted world has an easy mode too but for the money I would buy Mario odyssey.
My favourite game on Switch is Zelda breath of the wild but I'd definitely wait a few years for him to play that as it's a bit too complex.
I hope that helps, I'm sure I've forgotten something!
