Take a moment to relax, think and drift your mind back to the past - think back to your childhood.
What is the first memory that comes to mind?
As soon as one does, note it down and then free write. Don't try to force yourself to remember all the exact details.
Catch that memory, and then follow it where it leads you - let the words flow - even if you begin to write about something completely different or irrevelent. Keep writing. continuously. Do so for at least 10-15 minutes, longer if you have time. Don't analyse.
You may be surprised by the results. one memory can often unlock the doors to other memories that you may not have thought of before, or it could lead to something that relates to you today.
Follow up exercise:
Bookmark the page. Few weeks or even a month later, begin the journal entry with that very same memory and then free write. See where it takes you - one memory could end up leading you to totally different directions depending your mood, emotional state at the time.