IMO it depends. If i am there covering the event (went though the bid process with the coordinators, have a booth set up, etc..) I will take requests at just that, requests only. When i do them, i will mark the field as a high priority and 99% of the time get someone out there. But i will not guarantee that i will be there for the game the specifically wanted. What if an entire team come up and requests a game and then just one parent come up and requests another...you are going to take the one with more requests. But the thing is i will get a picture of them at some point during the day...if you dont, you are usually doing something wrong or need more staff to get all the teams.
Now, if they were just asking me to come out to a game and shoot, that i was not going to be at already, i would just charge a per-game rate. If you wanted to offer this at an event where you are the photographer already, i would reccomend hiring someone to do just team requests. But with the requests the parents are required to buy the CD of images for $150 (or whatever, but get $ ahead of time) from the game and you guarantee you will be there for the entire game...which is very cheap if they get the other parents in on it. Have a schedule for the photog, and once they fill the schedule, dont take any more of the guaranteed requests and just treat them as a priority field.
Hopefully that made sense