I have an iPhone and iPad whose power adapters are both two-prong. Since I cannot ground them in a surge protector, is it necessary that these devices even be used with a surge protector?
Grounding the 2 prong adapters has nothing to do with it. The point of a surge protector is
to protect electrical devices from voltage spikes. A surge protector attempts to limit the voltage supplied to an electric device by either blocking or by shorting to ground any unwanted voltages above a safe threshold
In other words, the idea of the surge protector is to prevent surges of electricity from ever getting to your device. As long as the surge protector ITSELF is grounded, your devices being ungrounded is irrelevant.
The chargers/adapters you're using are little transformers - they convert 120v AC power into a certain amount of DC power using magnetic conversion fields (wires wrapped a certain #'s of times around one side of the magnet for incoming power and a different number of times around the other side of the magnet for outgoing power). An electrical surge (say 1000V instead of the usual 120) into this device will, in turn, cause the magnet to convert that much more power - so when the output should have been 9v it could be as much as 900v.
So yes - absolutely - use the surge protector, regardless of the number of prongs your device has.