It is totally random. They generally look for families with kids that look like they are out to have a fun time. If you get there early, show some enthusiasm and talk to the CM's your chances are better. They generally pick that morning, but sometimes there are more than one family that they think would enjoy it and they will ask both, one for that day and one for the following day, so... You just have to get there early and be ready to have a great time. It is totally luck of the draw.