nunuvyer is correct. Cantonese is a spoken dialect. It only exists verbally. If you write it down or read text, Chinese is Chinese. The only difference in written Chinese (what is displayed) is Traditional versus Simplified. If your parents speak Cantonese, they likely grew up in Hong Kong or some place outside of mainland China so they would be used to reading in Traditional Chinese. Mainland China, under the Communists, taught simplified Chinese to their citizens. Everyone else (including Japan and Korea) learned using traditional Chinese characters.
Now, what you read/write in Chinese actually comes out different than what is typically spoken in Chinese. For instance, if you listen to a Cantonese newscast, the words and phrasing used by reporters is different from what you hear out on the street. It's more "formal" versus "casual" slang. "Formal" is basically the Chinese as it is written. That said, your parents should still be able to understand the iPhone just fine if you leave it in "Traditional Chinese". If you can read Chinese, you'll understand the "formal" spoken Cantonese. If they can't understand "formal" spoken Cantonese, then they're not going to be able to read a display in Chinese anyways cuz that's how it's taught. Mandarin Chinese is the same spoken or written.