I have watched a four year old play Breath of the Wild...it can be very simple in practice.
For sure, but that's not really the issue I would suggest.
A four-year-old can footle around hitting the easiest monsters (if it's anything but those, they'll be dying a lot, which I doubt they'd enjoy), or monsters made easy because someone else got gear/hearts on the character, and that'll likely be hugely entertaining to them, but I do not believe
@Reynard is four years old!
And any adult playing will be using their brain to some extent, whether they want to or not, which changes things, and causes you to be making assessments and plans and analyses a four or eight or maybe even ten year old is very unlikely to making (part of the reason adults are often insanely better at games than children or teenagers is that they can make those assessments too).
And fundamentally, BotW is a slow-paced, low-intensity metroidvania, where cunning plans and tricky trickstering are encouraged and rewarded (especially in the sequel), with a lot of platforming and (mostly easy but sometimes annoying) puzzles not really a jolly mindless hack-and-slash. Just walking around hitting monsters and ignoring goals/mechanics is something you can do in countless games - but it's making that part engaging to an adult (or older child) which is trickier.
Ever tried the Dynasty Warriors series?
Yeah now we're talking - that's exactly the vibe. Relatively intense but uncomplicated hack-and-slash, with no complex mechanics even hidden or buried. That's a strong suggestion!
@Reynard the genre as a whole are referred to as "musou" games and almost all of them are by the same company and called either [something] Warriors, with Dynasty Warriors, which is set in the Romance of the Three Kingdoms era as the original. There are a ton of them though, including ones based on Dragon Quest, Zelda, Fire Emblem, Persona, Berserk, and so on.
en.wikipedia.org