Riverside police shake and sway with school children as cameras roll. Their county counterparts, deputies from the bomb squad and coroner’s division, dance while pumping “JoinRSD.org” signs up and down. Back east, Detroit cops prance beneath the fist monument to boxing champion Joe Louis, and Cincinnati officers high-step with NFL players. These videos and dozens of others have been filmed and posted online this year as part of the Running Man Challenge, a cheeky effort that law enforcement officers say is designed to engage the community and show a side of police that the public does not normally see. The...