First a little background: My son loves rolling dice, so he has been "playing" RPGs with me for a good 6-7 months now. Although at the tender age of 3 1/2 he is far from being an avid role player. Anyway, he enjoyed rolling the dice, and basically playing out combat. It worked out really well for me, as I got into Savage Worlds around this time, and he was my always available player to test out game mechanics that were new and unfamiliar to me; I'm looking at you Mass Battles, Chases, Raises and Toughness. So while he helped me do this, and we even managed to create him a Character and introduce him into a little makeshift gaming group comprised of mostly newbies ( my little sister, my 12 year old Daughter, my Son and my old buddy through skype ), he never really got into anything other than rolling the dice, and uttering things such as "I bust that guy up!".
Second some alternate background: Roll back to a little over a year ago, Christmas, my son is 2 and just got a playskool castle with some Knights, a King and the Knight you see above. As soon as he opened it we proceeded to play with it daily, for a good month we were tromping on plastic horseback across carpet laden fields looking for...the Flag! Yup see the Castle had a single yellow flag that could be taken off and put back on. This same flag also happens to fit quite nicely in the hands of said knight pictured above. This is the infamous Ram-Man, the evil Knight who wanted the throne. However since he had no army, the best he could do is sneak in to the castle, and steal their banner, then fly away on his riding dragon ( yes the set includes a red dragon, that has pegs in his back for Ram-Man to ride on, as well as flapping wings and a biting motion ). This brought us hours upon hours of fun, as my son would chase Ram-Man around the house with his Knights firing Cannon balls, throwing axes, and just tackling me, to beat up Ram-Man, secure the yellow Flag and take it back to the Castle, all just to repeat the process all over again.
Now fast forward to about a month ago: My son asked if he could play "Dice" and I of course agreed, and we proceeded to pull down the dice, and the books, I handed him his Character, and prepared for another 15-20 minutes of me trying to find excuses for him to roll the dice. Then I looked over across the Table and saw him laying on the edge of the table. Ram-Man! It all instantly clicked. We have a setting! We have catch phrases, trademarks, backstory already to go! So a quickly tell my son, you know what would really make this fun! If we used the Castle and the Knights to play dice! So we quickly grab the Castle and the Knights and the story took over. Soon my son was all into character and even helped evolve the story!
"But Daddy, if Ram-Man doesn't live in the Castle, he needs his own Castle"
"But we don't have another Castle, Caleb"
"So lets build one!"
"Awesome! I'll get the Legos!"
So now Ram-Man has a Castle that we MUST build out of Legos before we play "Dice". And now my son plays RPGs for real.