Ah good catch, they must have rearranged this when they added Stage Manager. It's more obvious now with its own "Multitasking & Gestures" in the top level settings categories, right above Accessibility.
There are three choices now: Off, Split View & Slide Over, or Stage Manager.
The real trick with Settings though is that it has a search bar at the top of the sidebar (with no visual indication that it exists unless you find it by scrolling up past the top), so you can always use that instead of hunting through the many pages of settings.
Parent commenter should probably turn off the 4-finger app switching gesture and the 3-finger undo/redo/copy/paste gestures as well. Doesn't seem like a feature their mom will be learning to use on purpose.
In the current version of iPadOS 17, you can turn off Split View & Slide Over (like you describe) but for years after their introduction I could find no way to turn them off after my spending hours Googling and navigating Settings. And my Googling surfaced many comments claiming that there is no way to turn them off.
Don’t know what to tell you, it’s been in there since iOS 9 when multitasking was first supported. it was originally in a Multitasking section within General settings.
It was there in iOS 9, in iPadOS 13 (the 9to5mac link posted yesterday), and is there today in iPadOS 17, but I'll concede I'm not spending the time to rule out that Apple took the option away during the year you were trying to turn it off and then they put it back afterward.
