Anything that is related to the actual Sweepstakes (all the prizes that are listed in the offical rules of the sweepstakes itself) are completely random and a matter of being in the right place at the right time.
The Magical Moments that are being created is where you will see a lot of families and such because for a MM the Cast is picking the people. Magical Moments are things that are done to make the day better, but not a tangible prize in the sweepstakes, for example riding in the wheelhouse on the Riverboat or Waking up Tinkerbell, etc.
For the prizes, the Dream Squad is told exactlly where to go, when to be there and how many of what prize to issue. For example it will say Unload of the Tom Sawyer Island Rafts at 11:15am issue 25 Dream Lanyards. The first 25 people at 11:15am exiting a Tom Sawyer Island Raft will then be given the prize, no picking, no choosing, just the 25 people in the right place at the right time. The cast is not told where they are heading, what time they will be there or what they are issuing until about 10 minutes prior to the issue time, that way they can not tell people where to be (the sweepstakes follows very strict Florida and California State Laws to make sure there is not a bias od discrimination when it comes to the official prizes).