The children, 1 and 2-year-old Rialto residents, were with their parents visiting a family friend in the 800 block of North Garner Avenue at 6:10 p.m.
Police said the children were playing in the back yard when an 8-year-old pit bull bit the older child, causing facial injuries.
When the child was rescued from the dog, the dog then bit the 1-year-old child.
Paramedics took both children to an area hospital for treatment. Their injuries were serious, but not considered to be life threatening.
San Bernardino Animal Control officers have quarantined the animal.
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