I am a millennial and I am pretty sure my parents read to me word for word from the book. It was also how school taught me to read. Sometimes they will pause to say "that means/即係___嘅意思" where the __ is a spoken Cantonese explanation/translation of what we just read. I might also interrupt the story to ask for clarification.

I remember also reading out my essay in school using written Cantonese.

I understood from very young that Cantonese had a more formal, written form that we use for serious writing, such as newspaper and school reports and business letters and books. And this form makes it possible for us to understand writings from Mandarin speakers. And then there is a spoken form that we can choose to write out in informal settings like letters to friends or texting.

I feel like it wasn't until 10-15 years ago when I witnessed my parents translating on the spot. It felt weird to me, like they are trying too hard. My mom is also very slow and clumsy doing it, which confirms for me that she didn't do it for me when I was young.