While I've never done the Christmas parade taping at WDW (but will be this year!!) I have been an extra in a few other things. There is a lot of standing around, waiting!! From what I understand, if you are going to be one of the extras for the parade taping, you could be standing in one place for an hour and then see a float, then another long wait and so on. This is the reason I'm so glad I'll be solo this trip. There is no way my dd or dh would be able to do this. While my dd, 12, has done stagework and realizes how much 'hurry up and wait' goes on, it would be miserable at WDW.
Depending on how old your child is, it could be pretty boring after a bit.