Mini football camps are designed to introduce kids to the game of football. They're almost like a mini-version of the NFL camps in that the campers are timed in the 40-yard dash, measured in the long jump and vertical jump, and take part in the agility drills.
There are individual drills to increase player skills at their respective positions. So you have kids already learning how to catch, how to throw and so on. Camp can be as long as 1-3 days in length.
Camp includes room and board. Players and coaches get a pretty good idea of what the kids they're watching can do and what they need to work on, and they tell them that.