Fantastic! You have decided to create a valuable tool to help grow your young writer into a stronger young writer.
Step one was to get a three ring binder and dividers labeled with each writing style. (Simple definitions provided for your knowledge.)
It is now time to collect your first writing sample. Of course, you can assign any writing of your choice, but since this is your "Baseline Sample", you want it to be written with minimal help from you.
Before you assign a writing task, you must decide on the beneficial writing skills needed for your child.
You may want to refer to your child's grade level standards for direction with that.
For example, I'm having my fourth graders working on CCSS 4.4 in writing: "Produce clear and coherent writing in which the development and organization are appropriate to task, purpose, and audience." We are also incorporating CCSS 4.3, 4.8 and 4.10 as each writer will work on a narrative with a relevant experience adding revision and reflection of his or her work along the way.
So, the writers will be asked to write about a "Summer Moment" after filling a whole page with pictures and words of places, things, and people. From that pre-writing activity, the students will choose the one moment that meant the most this summer. The only directions given them will be to write and describe that moment to the best of their ability. That's our starting point.
Use a writing prompt that suits your child and supports the style and purpose you need to instruct. There is much to be found at www.k12reader.com, but one specific page worth visiting is: Expositive Writing Prompts - All Grades