Are ideas wanting in your homeschool? If so, you will find composition challenging. Composition starts with oral narration, copy work, recitation, and then dictation from the best writing available. These things continue throughout their schooling, adding new layers every year. Grammar is important, yes, but read what the beloved Fabre has to say about it in relation to writing.
Grammar cannot teach one to write. It teaches us to make a verb agree with its subject, an adjective with a substantive, and other things of that kind. It is very useful, I admit, for nothing is more displeasing than to violate the rules of language; but that does not impart the gift of writing. There are people whose memories are crammed with rules of grammar, who, like you, stop short at the first word.
Language is in some sort the clothing of thought. We cannot clothe what does not exist; we cannot speak or write what we do not find in our minds. Thought dictates and the pen writes. When the head is furnished with ideas, and usage, still more than grammar, has taught us the rules of language, we have all that is necessary to write excellent things correctly. But, again, if ideas are wanting, if there is nothing in the head, what can you write? How are these ideas to be acquired? By study, reading, and conversation with people better instructed than we.
-Jean Henri Fabre, The Story Book of Science, "The Book"
I am reading this to my 10dd and we are enjoying it so much. This particular, outdated chapter was about how paper was made from cloth rags years ago. "But isn't paper made from trees?"she asked. Which led her down a most interesting rabbit trail of her own. When it came time to narrate the above passage, she didn't skip a beat and spoke as someone who knew how true it was. She has only written a few narrations on her own, but she is not lacking for ideas!
Here is a little more about composition from an older post, Scintillations From Their Own Genius.
With affection and regard,