I agree with your thinking -- kids need a person to help them with writing. In other words, it's impossible to hand kids a workbook & expect them to become better writers. That's the first thing -- they need one-on-one adult guidance.
The second part is whether Writing Strands will give *you* enough guidance. Of course the standard answer is that it's inexpensive, so why not give it a try before you decide (knowing of course that the Hazells recommend it).
I'd like to give you a "preview" of how it works. My son is in the middle of Level 5, so this isn't an exact comparison, but WS really cycles through skills at gradually higher levels, so when you do Level 3 assignments they will be something similar but easier I guess you could say.
Ds's last lesson in WS was lesson 10. It was very short (3 days). Here's how *I* looked at it:
1. Carefully read together the things he will "learn" in this lesson. They're listed very clearly at the top of the page... "It may take you three days to learn..." This lesson is about writing dialogue and changing tenses.
2. Then we read the lesson. The first day is often just reading. There is time to discuss both of your interpretations of what you will be working on in the days to come.
3. The lesson pretty much always includes sample writing. A timid writer can basically imitate the sample. Don't be afraid to let your child almost copy it. Some kids will need to follow models for a long time. A fearless writer like my youngest can take the sample and change it to his heart's desire. He can change it even more than the book seems to "allow" -- as long as we can look back at the goals we read at the top of the first page and see those in his writing. Believe me, I've had both types of writers in the same family!
This particular lesson gave a "picture" of a specific situation (an airport with a time zone line through the middle) and the tense would have to change back & forth a lot of times (past, present, & future depending on where he's standing), and it would be all dialogue so we could work on how to write direct quotes and all of that. However, my youngest had his own "picture" in his own little mind, so he changed things around in the assignment. This was fine as long as (a) he kept the tense changing idea so we could work on tense and (b) he kept the dialogue situation so we could work on punctuating dialogue (and continue working on the skill from the previous lesson, where he learned that more advanced writing rarely uses "he said" or any other identification of the speaker, and yet it must be clear who is speaking at all times).
4. When I am evaluating his writing, I have several resources in WS:
a. Always go back to the little list of things being taught in the particular lesson -- at the top of the page.
b. You will be aware of what lessons have been previously taught (even in previous levels), and can provide reminders about those skills.
c. There is almost always a sample piece of writing in the lesson. Remember that the sample is there to help you "picture" what your child "might" do with this lesson. The book can be used with several different grade levels. The sample isn't there to criticize or intimidate. Use the sample to help you teach. And again, don't be afraid to let your child almost copy it, or to let your child swing far and wide away from it -- as long as the focus list at the top of the page is still being worked on.
d. In the back of the WS guide is info on writing problems. You can skim those pages and maybe choose one which your particular child could use some work on. Read through that one together and ask the child to edit their work with this skill in mind.
e. If you're still afraid you're under-teaching or over-teaching, WS does have a book called "Evaluating Writing" which gives examples of many different levels of writing & possible parent conversations about them.
f. There are tons of WS users on the MFW boards who love to chat