Some things are actually worse on the iOS side. It took years for Apple to catch up with spam and scam calls/SMS detection.
Plain Google search is still the main vector of scams, I eventually set up NextDNS on her devices.
I screen her emails with her consent, very easy to do with Fastmail that imports her Yahoo mail into a folder she doesn't see and then I move okay emails to her inbox.
If you do go for a smartphone, my experience tells me that there's no difference between Android and iOS. The biggest sources for shady apps are the Google Play Store and Apple App Store. Shady stuff on the web can be easily defeated using an adblocking browser, which is essential for older relatives.
With a well-supported hardware configuration and a working web browser, even a non-techie may have a more stable experience than they would with Windows.
That has as much to do with the decline of Windows as with the ascent of desktop Linux, but still.
Bulletproof, immune to all the apps and malware. She couldnt break it.
I recommend getting an Android phone (there are cheap Google Pixels out there) and try to sideload an app. Also browse the web a bit without an adblocker. I'd be surprised if by the end of the experiment you thought that sideloading is the reason their grandma's phone is full of crap.
Parental control is a also a hot buggy mess on iOS currently. Our daughter has an iPhone with parental control set up and a bunch of apps that are whitelisted regularly refuse to start at random moments (blocked by parental controls). We hoped that iOS 26 would finally fix it, but nope.
It doesn't really matter, both phone ecosystems are a mess, but in different ways.