Witnesses said a boy was using metal ladder to climb out of the O'Connor pool when he felt the shock. Two other children were injured seconds later.
A lifeguard told all swimmers to leave the water. Officials said the pool was being drained to try to find the source of the electricity.
"The pool will be closed until we figure out exactly what has happened here," Susan Slawson, a deputy commissioner for Parks & Recreation, said.
The children were reported to be in stable condition at the Children's Hospital of Philadelphia.