Honestly, who cares? If so, it would be a vandalism charge. How bad could that be? And I'm not being nasty here. If I saw a dog or kid in a car in the heat, I would have no problem breaking the glass. I might even throw the money at the owner and tell them to take better care of their supposedly loved animal. You wouldn't get jail time, our jails are so full even felons are getting probation.
That sounds like the more wise decision to make is to call 911 first so they would be aware of the situation first. If you do not call 911 first and you just go on and bash the window open then you'll be in deep trouble.I would call 911 first, tell them what you are seeing and say you want to break the window to save the dog. That way they have a record of what is going on and you have actually called in the emergency before damaging someones property. They would probably tell you to do it unless they had someone who could be there within moments.
I would like to get some confirmation on this, would it be a simple vandalism charge, or something more serious like breaking and entering? Most of the time people who leave their pets in cars are dicks anyway, what if they press charges even though it's clearly their fault?
I wouldn't. There are non-emergency numbers you can call and, at least around here, they're very quick to respond and help.