AAA membership - cheap insurance, with a cell phone you will be rescued roadside. Get the enhanced membership and they will tow you up to 100 miles. And if they don't use it, they can always use it with their friends (you don't need to own the car, just be on scene when the breakdown occurs).

Seat belt cutter or two - Countycomm has the Gerber Rescue Knife for under $10, seems like a good deal - http://www.countycomm.com/cutter.htm. Make sure they keep it somewhere within easy reach, like the side door pocket or even attached to their seatbelts.

Window punch - available at any AAA office, under $20. Also keep it in the main compartment of the car.

Signalling devices - road flares, red triangles, whatever floats your boat. Telling kids to get out of the car to warn approaching traffic by setting up some warning signals a decent distance behind them is situational and iffy, but I hear alot about folks read ending the back of stalled cars more often than I hear of people getting run over in the street while setting them out. And some folks don't like road flares as they risk igniting spilled gasoline, but they are cheap and hopefully your nieces and nephews know when to deploy? The non-flammable type of road flares (LEDs) still seem pretty pricey.