Our Gainesville, Georgia (about an hour away from Atlanta) dance is small, so keeping track isn't a big problem.
I have the person at the door do hashmarks for the different categories w/ detailed counts for familes ($20 family max).
After the dance I cross check the official door counts (hashmarks) with the $ in the cash drawer.
Often the hashmarks are less than the cash drawer by a few dancers.
So it is an approximation.
The count is important to me because:
    what is the proportion of students & kids (under 13) to adults
    are my numbers going up or down, etc.

My goals are a 30% under 30 (easy quip) or 25% or more students.
The 150-250 (or more) dancers dances in the SE with the majority being under 40 are my idols (e.g. Charlotte's dance or River Falls).
But I digress.

We charge $10/adult, $5 students (teens and up), $20 family maximum (cost friendly for large families)
with non-dancers and children under 13 free.  This isn't a true self-identified sliding scale but
the family max helps large families w/ limited income.  BTW, related to that, our dance is baby/child friendly.
We have a carpeted walking track around the dance floor where kids often hang out (wooden train sets).
Also common to see a child strapped to a parent who's dancing, or a child sitting on a parent's shoulders.
At our last dance 2 beginners we had 2 3rd grade aged sisters.
They did great and by the end of the evening they were dancing together (as a couple) in the line.
Worked well. They stayed to the end of the dance (11 PM).
The cute factor goes along way in making the experienced dancers tolerant.
Most of our dancers are very experienced dancers, many of whom drive a distance to come to our dance.

-Heitzso