Happily, you don’t have to create assignments from scratch. Various programs, including Sonlight, have assignments by grade and ability.
But a general progression might look something like this:
In early elementary, your children come up with creative storylines in their play, or re-enact a story they’ve heard or seen. If you are willing to act as scribe, have them tell you the story and you record it.
Fiction creative writing can continue as long as the children enjoy it. Not all children will. Some are matter-of fact engineers and scientists who don’t want to get in touch with their feelings or write about lost puppies. That’s fine. Most of the writing in the world is not fiction.