A little late to the discussion, but I'll add my two cents anyway!
In terms of F!, only the Blue Bayou Dining Package guarantees their guests ground seating for F!. Most reports indicate that the RBT viewing area and dessert party viewing areas also have ground sitting, though it's not guaranteed. In terms of the daily FPs, there have been some reports that guests near the RoA are allowed to be seated, but those spots go fast. Overall, the only way to know for sure that you'll be in an area that is exclusively ground sitting is to spring for the Blue Bayou dining package.
As mentioned by
HydroGuy, WoC is standing room only. It does not work well for shorter adults or small children. You would have to hold your child the entire time so they could see. They will not allow you to put your children on trash cans, railings, or any other structures that might be nearby.
Both shows are similar in length; WoC is 26 minutes, while F! is 22 minutes. Even with the FP system, it still sounds like you have to commit a fair amount of time to get a good view if you don't spring for a package (hour or more). In terms of scariness, it depends on the child and how easily frightened they are. Personally, I think F! is more likely to scare folks because of the dragon (which is pretty impressive, breathes fire, and is loud). WoC is less intense that way because it's all mist screen projections which are a little fuzzy to look at and there nothing physically "real" to see.
In all honesty, if you don't want to devote an hour or so of time and you don't know how the kids will do, I'd probably skip both shows. I don't think WoC is worth holding up a child for 26 minutes, or trying to keep them in the same spot for the hour that you have to wait to have a decent/okay viewing spot. I like F! better, but if you don't know if your kids will like it, I'd probably not spend the money on the dining package.