There are four parks, each with its own drop off zone. They can travel between parks if they feel like it and have time. So you could drop them off at one park and pick them up from another.
The basic principle is to go to one of the parks (there are big signs directing you to any of the four parks), pull up to the parking gates, and tell the attendant that you're dropping off (or picking up). They'll direct you to the correct lanes, and possibly tell you to tell the next cast member what you're doing so they can send you the right way. You'll essentially go to the same place as taxis, IIRC. At Epcot, Animal Kingdom, and Disney's Hollywood Studios, you'll drop off right in front of the park. At Magic Kingdom, you'll drop off at the Ticket and Transit Center, where the kids can pick up a monorail or boat to the park itself.
It's a little faster for the Magic Kingdom if you pick up and drop off at the Contemporary Resort, which has a walking path right to the front gates of the Magic Kingdom; no monorail or boat required. You just head to the Magic Kingdom, and stay right when you get to the parking structure. You tell the gate attendant that you're dropping off at the Contemporary, and he'll send you to the right and then you follow the signs to the Contemporary. Tell the gate attendant at the Contemporary that you're dropping off, and drop them at the porte-cochere in front of the hotel. They can get directions from any cast member to the walking path to the Magic Kingdom.
