Why do coaches insist on long distance runs for fitness? Ther longest soccer field is 125 yards or so.
The soccer player at the professional level runs about 8-11,000 meters in a match.
10000\25meters = 400 -25meter sprints in 80-90 min
25yd= 27.5 mtr 10000mtr = 109500 yds
So for us Americans about 11,000 yards.

Almost all of these runs are 10-25 yards in length.
So therefore, adding short sprint work to you session at the beginning(never the end) makes perfect sense.

You can make it fun as well as challenging. Relay races with 4 players on a team of a distances of 5,10,18,20 yds.
You get a 4/1 rest to work ratio, and the kids will run hard in order to win. You can do this as a shuttle run as well(up and back 5,10,15,20).

If you are sending your kids on runs longer than 40 yds with out a change of direction you are really not accomplishing the results your players needs.
Fitness should be part of every sesion, but at younger ages (9 and under) should be done with a ball.
IE: pass a ball out and they must run to go fetch it back. Again at different distances or to goal. No long lines 4-5 players in a line at most.