Too many kids
I thought the basic premise of the film was quite good, but there were too many kids involved. How could Vince Vaughn's character hope to act as a 'father' to that many young people, whilst also trying to balance his real family life and job, friends etc?
Also, there's no way that many young people would all get together and act so happily and group-hug each other as if they're in some kind of Bennetton advert. In a group of over 100 American young people there would be emos, goths, skater bois, preppies, jocks, valley girls, all the multifarious cliques of American youth culture who would not necessarily all get along like one happy family just because the same guy jizzed their genes into a paper cup twenty years ago. Instead, the group all seemed to be middle class liberal west coast college hipster types who all bonded like they were on Spring Break without the need for beer.
Also, some would be likely to get TOO close. Imagine the 'issues' that would be raised by some of those kids inevitably falling in love with each other.