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In my humble opinion I think a process approach is more effective when working with young learners, they might find learning grammar structures difficult at first so they learn by doing better. On the other hand, a product approach might be more adult based, people who need to learn how to write specific texts with specific vocabulary, people who need to sit official exams, etc.
I use guided writing activities, texts where there is information missing. Kids read models first and after reading them, I make them change some of the information and follow the model.