The other team has marched themselves all the way down the field and you now find yourself backed up and guarding your own end zone. This is not a time to be passive. This is the time to pin your ears back and be as aggressive as you possibly can. Your interior defenders should do whatever it takes to create pressure and get to the Quarterback. Your defenders in the secondary should stay as incredibly close to their man as they possibly can in order to either get an interception or block the offensive reception, because even the smallest gap in coverage can give the offense an entire score.
Your secondary will need to employ a man-to-man coverage because there is too much horizontal space to guard with such a short field. Solo Cornerbacks who find themselves alone in coverage on one side of the field may want to get inside leverage against the man that they are guarding in order to cut off an easy throw to the inside and force the offense into making a more difficult throw up and over to the outside.
If you allow the Quarterback unlimited time, then any moderately skilled flag football player will be able to eventually find one of his receivers open in the end zone through a gap in the zone. The only way to cover receivers in the end zone is to stick to each one and never give them an inch of space.
A stop in the red zone is a huge change in the course of a game because it is equivalent to taking away an entire score. Practicing this aggressive approach close to your own end zone can tilt an entire game in your favor with just a few successful executions.