how is Nintendo worse than other companies in that regard? Is microsoft or Sony better at the preservation than them in some way I don't understand? Are they better companies to support in that metric?
Both of them make PC ports now, which are automatically better for preservation, since PC is an open platform rather than a walled garden. Neither is perfect. They both put out live service games that are bad for preservation. But Microsoft especially is better at making their back catalog available for purchase years and years after release. Nintendo hunts down ROM sites and gets them shut down but won't make their old games available in a state that their potential customers are willing to pay for them.
As for the kid thing, they don't really get much pick in the matter of what game I let them try yet. It's just that I can't FIND any games outside of Nintendos stuff that is quality and fun for the family. It's mostly poor quality IP tie ins. I've looked!
Cassette Beasts (like Pokemon)
Lego <just about anything>
Overcooked and Overcooked 2 (it could be difficult for your kid at 5, but it might also be fun to play together depending on how they take to it)
Penny's Big Breakaway (upcoming, from the Sonic Mania team; 3D platformer)
Poi (I have no experience with this one myself, but I hear positive comparisons to Super Mario 64)
Sonic + All-Stars Racing Transformed
Sonic Mania
Do the kids still like Spongebob? There's that new Spongebob: The Cosmic Shake. That one's licensed, but I hear it's quality enough. It's also a 3D platformer.
Stardew Valley
Yooka-Laylee (admittedly, this one got middling reviews, but I liked it more than most people; 3D platformer)
Tearcell