The child does not need to have a ballet background, be it boy or girl. All the children will start basic ballet training from Year 1, even if they already have ballet training. This includes the boys. My daughter did not have ballet training prior, she auditioned with a Chinese dance piece, and she was only doing it at the CCA level at her primary school.
It’s best to prep early. For my first child, we started late with less than 6 months of prep and she threw away all her drafts and experimentations. She ended up with scraps that we could find to put into a sketchbook. My second child started much earlier to create 10 artworks based upon observation and imagination.
I believe that for this art form, the best way to prepare is to cultivate a love for reading. Our daughter did not enroll in any writing class / workshop outside school, but she loved books and read a lot. This exposure allowed her to find her creative voice, tell stories from a unique point of view, and think/write outside the box. As for the portfolio, I suggest that you keep the pieces that she writes in her spare time as a hobby, and the essays from school that stand-out. They will come handy. My daughter’s portfolio included past works from school and personal hobby, and new ones written especially for the SOTA application. It took her about 4 months to put-together everything.