Interesting questions....if you're looking for a place where you will see just about every kind of costume than I would recommend West Hollywood. The atmosphere is energized, a lot of people and a lot of celebrating.
However, if you're looking for a more relaxed feel, Santa Monica, Huntington Beach, and some other coastal areas have fun activities and events.
If you're taking your chidren, I would recommend a child friendly venue/event that is hosted by local citie government/s and that are advertised as child friendly.
Yeprem P. Davoodian, M.A.