I reckon it was definitely 'In The Deku Tree' I mean come on, its easy even in the master quest aha. also the dodongos cavern was fairly easy, but to be honest, i got lost inside jabu jabu's belly, couldn't beat those bloody tentacle things that are moving around in those rooms aha c:
If you don't think 'Inside the Deku Tree' is the easiest kid dungeon, than I can no longer help you. It's the tutorial dungeon of the game, and so easily broken. I've seen people beat it, and I've beaten it, in under 2 minutes. That is the easiest dungeon in Zelda history.
The Great Deku Tree. The map wasn't really needed since the dungeon was pretty straightforward. Run, get the slingshot, obtain a code, burn a few webs, and defeat the boss. Its not my favorite area, but it was the easiest to finish.
I definitely have to agree with everyone that Inside the Deku Tree was the easiest. It was the first one and the first major fighting that you do in OoT. So it is obviously a tutorial dungeon and is very simple to complete.
I'd say either Deku Tree or maybe Jabu-Jabu. I got stuck in the Deku Tree because I thought you needed to use fire on the first spiderweb you run into (that one on the floor that you just jump at and break). XD