A six-week-old girl and her two-year-old brother were trapped in the vehicle, in Luckley Road, Wokingham after a problem with the central locking.
Firefighters placed Packex, an adhesive foil, on a window before smashing it. The foil ensures the glass does not shatter inside the car.
With the window removed, the children were reunited with their relieved parents.
The incident happened at about 4.30pm and it took the fire crew about five minutes to free the youngsters.