The debut issue of Gygax magazine (a reborn version of the classic Dragon gaming mag) carried an article I wrote explaining the variant D&D rules my then-four-year-old daughter an I were using. It involved a blend of random toys from the living room, painted D&D miniatures, dice, and pennies from the piggy-bank for scorekeeping.
Now, one of Gygax's readers has posted his experience playing the game with his own daughter. He used a set of My Little Pony toys (including an awesome MLP castle) to build a campaign called "Assault on Equestria" and it sounds like his daughter had an amazing time -- as did he! It's been a while since I've played D&D with my kid; this makes me want to go dig out the dice-bag!
Movement rate was determined by rolling a d12 each time and moving up to that many inches. For having wings, we allowed the pegasi, Princess Celestia and all the dragons to add two inches to their movement rolls. Let slip the ponies of war!
Fighting consisted of a melee attack and was an auto-hit situation as long as one was reasonably adjacent to the target. Just roll 1d6 damage, except in the case of the wyvern (which had a stinger instead of dragon breath) and the extra-tough earth ponies (who needed something to offset the lack of wings or magic). These could all roll 1d8 damage in melee.
Next to be addressed were the magical abilities, used by the unicorns and Princess Celestia. I followed the suggestions in the article and initially we agreed on two spells, fireball and "zap" (or magic missile). The theory was that fireball was easier to hit with but did less damage than zap. In reality though, fireball was never used, and we ended up using the zap guidelines exclusively for any magical attack, as well as for dragon breath. To do damage, we first needed to hit on a d20 attack roll, throwing greater than or equal to the target's range in inches. A hit did 1d6 damage.
Lastly, the magic users could choose to spend an action healing other characters. This could be done at any range and restored 1d4 stamina. We didn't keep track of negatives; once a pony or dragon hit 0 they just fell over and we took the pennies away. So waking up knocked out characters was easy. I chose two of my six dragons as captains who would also have this healing ability.
ASSAULT ON EQUESTRIA! (Thanks, Jayson!)