Chris Bucholz in Cracked:
Use an appropriate technique. Modern mixed martial arts are geared almost exclusively towards one on one combat, and are not designed to take on multiple tiny aggressors. As a grown adult, you could be fairly assured of absolutely destroying a 7 year old if you took him to the floor for a ground and pound. But by doing so you'd expose your back and head to his peers. Your best bet is to stay on your feet and use striking techniques. Karate is one good choice – it was originally designed in the 1600's for use by unarmed Japanese day care workers.
Be aware of the terrain. By default, you're going to have a height advantage against twenty children, but be sure you don't cede it. Avoid fighting around picnic tables, monkey bars, or anything with which a particularly daring child could launch an aerial attack. The ideal situation is fighting children who are trapped in a ditch below you.
Stay mobile. Unless you're extremely lucky and find yourself fighting twenty infants, you're going to be at a mobility disadvantage when fighting a large group of children. You must avoid becoming surrounded at all costs. Keep moving, and always trying to position the bulk of children on one side of you. Circle, sidestep, and use tactical retreats to try and engage a single child at a time, where your reach and decades of muscular development should prove an advantage.
More here.