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.
The Animal Care and Control Team of Philadelphia (ACCT) told Action News that the owner surrendered the dog and the dog was euthanized on Thursday night.
The dog will be tested for rabies.