Is it safe to use the 12W iPad 4th gen power adapter for earlier iPads and iPhones? The older iPads have 10W power adapter (charger), and the iPhones supposed use only 5W chargers.

But the iPad 4th gen, probably due to the larger power consumption, uses a 12W power adapter / charger.

Is this charger safe to use with the older iPads and iPhones.  Will it overcharge or overheat or damage the battery?  A more serious worry is that if it overheats, then can the battery explode?
Yes. The wattage ratings are maximum and all iOS devices will only pull the current they need to charge. 

Should a device or an adapter get too hot to operate safely, they will shut off.