The collision happened about 2 p.m. near the intersection of Mountain View Road and Valley Boulevard.
El Monte police said the driver was trying to park in the Walgreens parking lot when they accidentally hit the accelerator, jumped the curb and struck a bus bench where the three victims were waiting.
The baby girl was taken to the hospital, where she later died. The baby's 36-year-old grandmother was also struck and was at the hospital in critical condition. A 12-year-old boy, whose relation to the other victims was not immediately known, was hurt, but was in good condition.
The driver was not expected to be arrested and there were no charges pending.