I think this is a GREAT idea, especially for young (ages 10-14), impressionable roleplayers with an adult DM. Having them play themselves, adventuring in the Land of Narnia, is particularly wonderful.
Simplifying the 3E D&D rules for this is essential. Here are some ideas, right off the top of my head...
* Get rid of AoO's and the rules for bleeding.
* Devise some simple alternative for the system that uses partial actions, move equivalent actions, free actions, etc.
* Instead of a PC dying when he reaches -10 HPs, have his Narnian self disappear and the player reappear as his true self, back in the real world. For young players, a PC's "death" should only be a minor setback for the group and/or a sort of a "timeout" for the player.
* Don't use alignments. Instead, the players should handle moral situations and make choices based on how they, themselves, would handle and make them, if they were to encounter such situations and choices in real life.
* Since the players will be playing themselves, the only PC race to be used will be humans. That's simple enough.
* It's probably best to use only the following PC classes: fighter, rogue, cleric, and maybe bard.
* If using clerics and bards, go through their spell lists and omit those that do not fit in with the Narnia setting.
* When the players enter Narnia for the first time with their characters, have them pass through some sort of keep or castle, where their starting equipment is laid out for them on tables and racks. (As I recall, one of the Narnia books somewhat started this way. Was it "Prince Caspain"?) Throw in some equipment that has no immediate or obvious use, but will come in quite handy, somewhere down the road, within the first game session or two.
* Someone else's idea: Condense the skill list, and set standard DCs for all skills (10 easy, 15 average, 20 hard).
* Again, someone else's idea: Keep PC levels low (1-5).
* As DM, declare that when PCs advance beyond 5th level, they have "grown up", and can no longer adventure in Narnia. (This is in keeping with the spirit of the books.) If players want to continue beyond that, they must play themselves again, only this time as different character classes, starting again at 1st level. When the players return to Narnia with their new characters, a considerable amount of time should have gone by in Narnia, introducing them to a whole new era.