I have a three year old little girl (she'll soon be four) and she has a good imagination. Naturally, we'd like to steer this in the right direction, so I've been GMing once a week with my family. Right now it is just a homebrew system that we're keeping quite simple. We'll be moving up as we go.
Our first hero is Drella. Her character class is definitely Princess, and while her normal tactic is to use either scorching ray or fireball, she also tends to pull a sword out of hammer space when things get rough.
And this is Ella, my wife's spellcaster. She's very helpful on adventures
Last is Prince McGuffin. He mostly gets kidnapped by ogres, sorcerers, dragons and such.
Our heroes have noted his aptitude for being captured, and have started keeping him from going out by himself for any reason.
Last time our heroes sallied forth, they started out by locking him in a closet until they come back.
I expected this campaign to mostly feel standard (we kill the monsters, save the prince and everyone is happy). But then my little girl realized that some of the monsters can talk- and if they can talk, they can be talked into doing things for them.
My favorite was when they convinced an ogre that the dragon would be tastier than eating them, so he came along (all the time threatening to become too bored or hungry and start eating the party).
Here are the three of them together.