There are many things that could be considered to be a parent's worst nightmare, but your kids locking themselves in your car is probably on the list. Time is a really critical factor here, although older kids can be instructed to unlock the car when it becomes evident that you don't find the situation in the slightest bit amusing. Here are a few likely scenarios and what you can do.
The Easiest Option
Depending on the age of the child, you might be able to convince them to unlock the car's doors of their own accord. Although of course it can be difficult to plead your case through a closed window. A spare set of keys for the vehicle is the next logical choice, but this is only practical if the incident occurs when the vehicle is on your property; meaning the keys are in easy reach, or if a spare set of keys can be delivered within a timely fashion. And time can truly be of the essence.
Parked in a Monitored Area
If not on your property, the location of the vehicle can play a big role. If it happened in a car park, contact the authority who oversees the facility. The telephone number for security can often be displayed throughout some shopping centre car parks, for example. They might have locksmithing abilities, allowing them to gain immediate access to your vehicle. They might also have a locksmith on call.
Calling for Assistance
If the vehicle is parked on the street or in an isolated area, you have a couple of options. Are you a member of a motoring association? Call them immediately, and their roadside assistance team will be with you as quickly as possible (with the ability to unlock the vehicle). Alternatively, call an emergency locksmith. In these cases, stress that a child is locked in the car and that the case is in fact an emergency.
Breaking and Entering
In an extreme case, it can be necessary to break the window yourself. This is when the child is in jeopardy, such as if a locksmith cannot reach you quickly enough and the temperature inside the vehicle is at a dangerous level (or will quickly become so). Choose a window that is furthest from the child. Find an appropriately heavy object, such as a rock. Protect yourself as much as possible. Wrap your hand in whatever is handy, such as an item of clothing. Turn your head away while breaking the window to prevent injury to your eyes. When the window has been smashed, reach in (with your covered hand) and unlock the vehicle.
Hopefully your kids will never lock themselves in the car, but quick action will fix the problem (ideally without needing to smash the window).