Just watched a news segment on what to do if you see a child or animal left inside a hot car:
Use a tire iron or get an inexpensive window breaker (less than $20 at an auto store). If using a tire iron, hold the curved part and use the straight end to smash the window.
Smash the window in the bottom corner WHERE THE DOOR LOCK IS LOCATED. (The window is strongest in the middle, so don’t even bother trying to smash it there.) Be sure to smash the window furthest away from them so they aren’t hit by broken glass.