itch.io

It's not too hard - Steph has played it with her 3 year old and it went swimmingly!  The key is the downloadable Chapters and just asking your kid questions - littles tend to love the coloring-book illustrations in the print n' play (and the accompanying AP podcasts are great, too!).
I haven't laid it out for A5, yet, but would love to.  It's a very minimal version of a slightly more complex engine, which is what I use in my educational classes (the full version includes "Drama Clocks," "Story Points" earned by writing that allow for development, and "Special Abilities" that allow certain rolls to be improved once per game).