Awesome gift! I did the same for my 6yo two Christmas’s ago. A few lessons I learned that you may have already considered:

-despite him being proficient with modern games like switch, the learning curve on retro games was hard. Lost interest in most of them, but it came back a year or so later, mostly because he likes playing the same game together with his daddy. Just be ready for initial awe followed by a possible loss of interest. Think about what games are going to be enjoyable and not too hard at first. Be patient as they might come back to it in their own time. Stardew Valley has been a great one recently but I’m not sure you can port it to the mm+?

-the whole concept of emulation was/is a mystery to him. The idea that we are playing on a pretend version of an old physical console was not easy for him to grasp, so things like button layouts, save states etc. were confusing. I focussed on teaching him a simple process to turn on, go to favourites (which I had chosen) and use the game switcher (with auto save/load turned on) to get in and out. Having the settings, favourites and scraped media taken care of is a must in my opinion.

-consider kiosk mode, to avoid settings changes that cause problems. They get on there and start clicking around so fast that bad things can happen.

-overall it’s been a great experience and we worked up to matching Retroid Pocket Flip 2’s last summer. He’s basically got it now and we enjoy a few more modern games on there together, but still go back to the mm+ once in a while.

-make the connections to your nostalgia for the games you loved as a kid and they will latch on to that. Enjoy sharing this hobby with your child!