A substitute teacher picked up a broom and went berserk in a first-grade Washington Heights classroom yesterday-whacking children in a wild melee that sent 20 kids to the hospital bloody and bruised, cops said. Mom Beatrice Balbuena comforts Angel Rodriguez, 7, who holds bruise on his head he received ... Danilo Nunez, 59, freaked out after he waded into the middle of a crayon-tossing fight between pupils in his class in Room 101 at Public School 4 on W. 160th St. at Amsterdam Ave., police said. The children screamed for help as Nunez swung the broom, pummeled them with his...