Day 4 of the Storytime plans focus on early writing practice, emphasizing names and simple words.

The lessons usually have you write a sentence, have the students write some words within a sentence context provided, and have the students draw/paint/trace something to go along with it.

I would just have your ds write the first sentence you'd usually write, as well as do the rest of his part of the assignment. The latter assignments starting with Unit 12, Day 4 have more writing than the earlier ones.

I actually had to help collect and compile Kindergarten assessments when I taught ps. Many of the dc were just able to write their names. The very best samples included a sentence and a little picture of some sort. I think by doing a sample with 2 sentences that are related and having some accompanying artwork, that should be more than enough.
Your wise words here are so true...
My understanding of this is that we want them to do copywork and copy other "great" sentences so that they can learn proper grammar, punctuation, spelling, structure and such from great writers. So my 1st grader is actually using ARFH A this year and he is copying scripture. (Obviously great writing.) If they write their own sentences, the incorrect spelling might get stuck in their heads and such.
After 7 years in ps of correcting students' writing within standardized testing, I can tell you that the single worst thing dc can do for their spelling, grammar, and mechanics is be told to free write with invented spelling in the early years. It is far better to do copywork of correctly spelled and structured sentences, that are worthy words to write, than randomly free write. Dc who have a CM education from early on can see what "looks right", and they have very few poor habits to correct.

I remember correcting a standardized writing sample and wondering for a moment if "frend" was spelled right. I'd seen "friend" spelled like "frend" so many times, it really did appear correct to me.

Yikes! Writing words improperly spelled makes spelling them improperly a habit. Please be encouraged - your ds is going to have good writing habits in place - and very few poor habits to fix using CM's methods. HTH!
In Christ,
Julie