I used to play a game with my kids where we’d each start with a piece of paper. One person picks a subject or a theme (eg dinosaur, alien, superhero, Halloween, fairy). Or one person picks a mood or vibe - eg living in an ocean world or jungle; magical; etc
First each person draw a head (or heads!) with a neck, and folds the paper over so just a bit of neck is showing.
Pass your paper to the person on your left.
Now draw a body and arms/wings joined onto the neck. Fold again, leaving just a bit of the waist showing.
Pass the paper to the person on your left again.
Now draw legs/tails/whatever bottom half you like.
Pass the papers one more time, and unfold the paper.
This gets the creative juices going and the moment of reveal is so funny! At that point - you can stick your creature in a notebook and write about it.
Sometimes we would just do a “fact file” - name it, imagine its habitat, its diet, whether it’s a mammal or a fish or a reptile or a humanoid, its special powers or unique abilities, and so on. In time you get pretty good at inventing ideas and writing about them.
Sometimes we would write a whole story together or dc would have a go on their own. Kids age 6 rarely finish stories on their own!