It is an after hours party that is from 7pm until midnight (but you can get into MK anytime after 4pm with your party ticket). They run a specail Halloween parade at 8 and 10pm and Special Halloween fireworks at 9:15pm. It costs $31.95 (plus tax) for adults 10 and up, $25.95 (plus tax for children 3-9 and free for 2 and under. There is trick or treating, the rides are open, the characters are out.. it really is a lot of fun. The rides are open so bringing a 21 month old would be just like bringing them any other time (as far as what they can do). But I am sure your child would really enjoy it, they can dress in their costume and trick or treat while playing at MK. The remaining days this year are Oct 14, 17, 20, 22, 24, 26, 28, 29 and 31 (but the 31st is sold out).
Crowds are not bad at all, we went last night and we waited about 5 minutes for Buzz Lightyear and that was the longest line we encountered (the lines may be a little longer for things like Dumbo, Winnie the Pooh, ect). But there is a limited # of tickets sold for each night so it isn't really that bad.