For our kids, we're blessed that they have relly good imaginations. We try to reinforce this all the time. Rowan's ALWAYS a princess this or princess that. And she's assign roles to me, her brother and her mom, deza. They she'll direct us in what we have to do...yes, a little GM in training.
Also, we try to watch as many things as possible and let her pick movies out, Rowan's favorite movies include Terry Gilliam's The Adventures of Baron Munchausen, anime like Tenchi Muyo, and musical stuff like Fantasia 2000. Sh ealmost always makes up a story to tell her younger brother or us.
Speaking of her brother, he's on my lap having me help him "read" a picture book.
She wasn't gamed, but when we were larping a lot, we took her a couple of times to watch our friends dressed up, fighting monster, and saving the day. It really helped athat a lot of peopl would play their characters over the top for her. It would make her day. Shame there's nothing kids friendly down here in Florida.
That's just a beginning, so what has other people done with their kids? Both Deza and I want our kids to enjoy gaming as much as we do and no lose their imaginations like it seems so many kids nowadays do.