I started playing with my kids when they were around 5 or 6. They loved it.
But, remember that it’s a game for children. You want them to have fun so they’ll play again. They can learn the deep-rules later. Don’t overwhelm them with rules. Think of it as like playing cops-n-robbers or whatever, but using dice and figures and maps. The goal is that they have fun. I suggest using a map and little figures, to help visualize. I suggest a simple quest: save the princess from the ogre, or kill the witch, or whatever. My kids, in fact, loved variations on fairy tales – I had them go through a giant’s castle, a la Jack and the Beanstalk, and included such wonders as finding the giant’s (slightly used) handkerchief. I also had them find a cannibal witch’s cottage, as in Hansel and Gretel, but changing things slightly (house wasn’t made of gingerbread, f’rinstance.)
Don’t spend a lot of time having them roll up characters the first time around. Make it simple. Use three or four “attributes” – strength, intelligence, endurance, wisdom. Let them start as fighters, it’s easiest, and perhaps give each of them a magic weapon to start off… and not just a +1 dagger. You want fun stuff, like a talking axe who’s very boring, or a cloak of invisibility.
In fact, when I started with my kids, I stuck with 6-sided dice, and I’d say, “To hit that ogre with your sword, you need to roll higher than 2.” Ignore all the stuff like dodge and attacks of opportunity and complications. Keep it simple, keep it fun. Make sure they win – they can suffer an injury, or loss of weapon, but make sure they win in the end.
Almost thirty years later, my son is still an avid player (he plays more than I) and brags that he’s second generation. So, enjoy!