Slow double-take: you meant "playing field," not "playground." ("Pitch" is UK usage). A "playing field" is the surface on which athletes play team sports, and it's called a "field" because it's usually covered with real or imitation grass. A "playground" is a facility, perhaps part of a park or schoolyard, that's equipped for outdoor games. One might say "the playground at Jones Elementary School has swings, a jungle gym, two basketball courts, and a soccer field."

A U.S. football field--American football, with the elongated ball--is 100 years long. The fifty-yard line is the center of the field. The best seats are the seats "on the fifty-yard line." Literally that would mean the seats were on the field itself, but it's customary to say "on." When describing where a seat is, you would want to know which line it is on so that you would know how good the view is. In a theater you might want "front-row seats," in a baseball stadium you might want seats "behind home plate," in a football stadium you might want seats "on the fifty-yard line." I'm not a football fan but I looked up some seat prices and I see that at one college game seats on the lower level, visitor's side are selling for about the same price on thirty-yard line as they are on the fifty-yard line, so I guess seats on the thirty-yard line are good seats.

It probably means the seats in the stadium (which you need a ticket to enter) are located opposite the 30 yard line on the pitch.