Just write it! I was writing poetry and oped pieces until about 8 years ago when my son (then age 7) comes home from school and tells me I have to write stories about his classmates favorite things. Puzzled I called his teacher and she informed me that during, "what does your parent do for work", session my son stands up and tells the class his father is the best writer in the world. I had one week to write stories for 16 kids. I finished them in time for my then wife to read them to the class (I can be monotonic). Seeing the look on those kids faces hooked me and I have been writing children stories ever since. I would have never thought I could or wanted to until my son put the pressure on.
Let your imagination loose, think not as an adult but look at the world through your eyes at a certain age from your growing up. You will be amazed at the stories and ideas that will come. Check out your local library for children book basics this will help also along with a little web searching can get you started.