The incident happened around 6:15 p.m. in the 3300 block of North Water Street.
Authorities said a 12-year-old girl and a 3-year-old boy, who are siblings, were playing outside when a pit bull ran from a nearby home and attacked them.
"It appears the dog got out of the house somehow, ran out of the house, not on a leash, and bit these two children," said Chief Inspector Scott Small.
Both children were taken to a hospital, where they were treated for injuries to their arms and faces.
Animal Care and Control Team of Philadelphia (ACCT) is now involved in the investigation.