With all the stuff most kids know it really surprises me that there are those who don't 'know it all' yet when they come to kindergarten.
Of course, those who have been in preK are so much better prepared than those who have not had that opportunity.
Just from what I've seen with my bunch, I've often felt that going by age &/or how long they've been in school is not the best criteria for placement of kids in a class. I know one of mine was really past the majority of the first several months of a particular grade, but not yet advanced enough to skip the entire grade. If there had been a 1/2 grade step up available it would have been ideal for him.
This same concept fits in with your 'what kids need to know before they start kindergarten'. Perhaps offering a summer program that would cover what is usually in preK...colors, shapes, letter and number recognition, using scissors, staying in their chair except when working in the centers, taking turns, responding to questions appropriately, etc.