Context writing in the examination is tough - there are no written intentions.
BUT, you still have to consider your target audience, your purpose and your contention, as well as the type of language that would suit such audiences and form of writing.
If the audience and form of your feature article allows you, or needs you, to be slightly 'informal' in your vocabulary - then this is absolutely fine... as long as it does satisfy your purpose and this distinction is made very, very clear to the examiner.
Do a FLAP+C+C plan before you write your context writing, and you shouldn't have any problems with your writing in terms of the literal choices you make. But be careful with your form of writing and vocabulary use - writing a children's book, whilst it necessitates babyish language and simple ideas, will not satisfy the proforma of the year 12 English curriculum by a long shot.
But I'm playing it safe - doing a traditional, expository, formal essay.. less can go wrong here.