This first session will help you to decide who you want to write for.
Think of a time in your childhood when you were very happy. Now tell the story of why you were so happy that day to a child. Imagine the specific child to whom you are speaking. It is not you the adult speaking, but rather the child you were then or perhaps even a good friend of the child to whom you tell the story.
Write for about twenty minutes or until you have finished the story, whichever you prefer.
Now read it back.
Who is narrating the story? How old is s/he?
Who is listening? How old is s/he?
If the two children are different ages, which one appeals to you most? That may well be the child for whom you should write.
However, it may be a good idea to try out all of the exercises in the following posts. You may yet be surprised by another idea.
Now start reading a lot of children's books. Join your local library. Follow the hash tag #introtochildrenslit on Twitter for lots more ideas.
Think of a book that was a favourite when you were a child. What can you tell us about the characters? The setting? The plot? What made you like it? What can you learn as a writer from that?