in the future, at least put a high value resistor in series. Chargers are usually 5v, lithium batteries get VERY unhappy with anything past 4.2 (4.3 with newer ones) and can blow up. with no current limiting, the "blow up" stage could arrive quite fast. Also phone's various regulators that power FROM the battery might not be rated to handle 5V inputs
That's generally good advice but then it would have taken much longer to get to the point where the phone's charging circuitry could take over and the battery being near empty likely served to peg the voltage at something really low. Finally, those chargers are usually not capable of more than a few amps and given the internal resistance of the batteries at that point the bigger risk would be to fry the charger if it can't handle something suspiciously close to a short circuit.
If you're going to use a resistor use something like 100 Ohm, that way you will still be able to see some effect of your action before Christmas and if you have access to a volt meter connect it parallel to the battery to see what's going on. As for the 4.2V, that's a single cell, some phones/tablets will have a couple of those in series so you can't assume that you'll find anything up to 4.2V at the battery terminals, checking that should be your first stop before attempting any of this.
