One of my children is very interested in filmmaking. He's always been interested in really good films - like "Sound of Music" and "Mary Poppins" when he was very young.
He creates storyboards now, even though he's still so young. I think it's great for kids to really use their imaginations and create things that interest them.
He and his older brother spend a great deal of time drawing characters. How to draw books and videos have really helped them. One great resource is how to draw caricatures. Here's a good resource
for caricature and cartooning.
For the most part, just providing the kids with a ton of paper and lots of markers, pencils, crayons, etc., can go a long way toward encouraging their creativity.