If your relatives are significantly tech illiterate, I'd skip the smartphone entirely and go for a locked-down Linux desktop + feature phone. The most dangerous apps are big legitimate ones.

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.