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.
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.
You want this fight to be over fast. Children have boundless amounts of energy, and you'll tire quickly as the fight progresses. If you schedule your fights with twenty children in advance for some reason, I urge you to focus your pre-training regimen on cardiovascular conditioning and snorting eye-wateringly large amounts of cocaine.
Although I don't expect you to be intimidated by the prospect of fighting twenty children - given the self-confidence that comes with maturity - remember that intimidation is a two way street. Twenty is a big number, and if that many children lose their fear of you, watch out. Use fierce roars and displays of strength to frighten the children. When taunting, remember that children are almost comically stupid, and won't understand any of your more creative taunts. You won't intimidate anyone if you have to explain three times specifically what you did to their mother last night.