Free your mind. Watch cartoons --- the sillier, the better. Write a list of images you see and of ideas that come to you while watching the animated worlds children would populate if given the chance.
Play word association as suggested by the Writing with Writers website (See References). Use your list as a creative jumping off point. Write words that you associate with the words on your list. Think of words that rhyme with the words on your list. Add those words to your list.
Tap into your creative brain. Turn your paper over and doodle words and images as they come to mind. Use crayons and colored markers.
Make a color copy of your word list. Cut out the words and paste them into a poem in any order you like on a sheet of colored construction paper. Give your poem to a child who will be the best judge of your poem for children.