If one is truly evaluating, you will not leave it up to kids to sub themselves in, telling the subs to do it themselves at this age tells me that they weren't even evaluating the new ones. If one has experience coaching/working with this age group, knowing new kids will not be assertive you are leaving the entire tryouts a waste of time for these kids. You would know having enough to evaluate is not the problem, a seasoned coach/evaluator can handle it easily.
We were at one of the "Crooked Arrow" auditions for kids and they did a better job evaluating 70 kids per age group.