I’ve tried doing this.
You might try Assistive Access on iPhone and iPad, but the passcode and the account and the mail and the rest will all be an effort to maintain, and — if my experience here is any guide — will all get tangled. No equivalent to this on macOS..
And the scam emails and texts and calls, oh my is that not fun to deal with.
Best bad option I’ve met is supervising the devices, probably with Apple Configurator app, but that’s still a project.
Supervising gives you control of the settings, accounts, contacts, app store, and most other details.
Stuff I had ongoing issues with: changes to mail settings. Changes to device settings. Spam and all sorts of scams.
Ensure Time Machine is running in the iMac too, either to local storage or to a NAS box located somewhere else on the local network away from obviousness.
Bringing the Apple Account into Family Sharing might appropriate too, as that can provide some sharing functions, such as iCloud storage, and sharing photos and such, and Find My. Family Sharing does not share everything.
Joining Family Sharing is necessary if you age-restrict, but deliberately mis-setting the age to a youngster likely summons age-related verification and access issues. I wouldn’t mis-set, as I’d expect about as many problems with parental controls as benefits. Or more.
One minor thing that did help with the email scams and email rubbish was a set of macOS mail rules that immediately deleted any messages with any attachments arriving from unknown senders. Things didn’t quite get to the point of dropping all messages from unknown senders, but it was definitely headed there. Here are things the
That iMac can help here with things that the other devices can’t, not the least of which are continuously-available mail rules, and well past what is available using iCloud mail rules or shortcuts.
Some of what Apple has available: https://www.apple.com/accessibility/features/?cognitive
And whatever you try here will be frustrating and contentious.