We did this last year, and will be back again. It's a lot of fun.
Disneyland does not give out candy. Maybe on Oct 31, but not on the rest of the days. Don't fret about candy though, you will have a ton of it. There's 15-20 trick or treat stops during the party and you get a handful at each. Last year I didn't even buy candy to give out at home, we had so much from our Disney trip.
One word of caution about heading back to DL after the party. If anyone over 9 (maybe 12) is dressed in costume, they won't let you in the park. We thought about having a change of clothes in a locker, but we were so pooped after the party that we just went back to our room.