They don't give an input impedance because it doesn't have one in the traditional sense. It periodically charges a sample-and-hold capacitor: during a three clock tick sampling period, input impedance is very low ~5K; the rest of the time, impedance is effectivly infinite. So the net current drawn by the ADC inputs depends on the sample frequency, and the convertion accuracy depends the host clock and on higher current during the sampling period.
For your thermistors, bandwidth doesn't terribly matter. So the easiest with them would be to put a 10nF cap between the input and ground. The cap would supply the higher currents for the sampling phase, and the net current through your thermistor voltage divider could be kept low enough to not affect the results by cutting the sample rate to 10/s or less.
If you need to measure thing with high bandwidths, you'll need voltage followers on the ADC inputs.
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