Agree with rocky and tm. Also, if they want this to call home to the US they should use Skype instead (via WiFi, which is free in most hotels, McDonalds or Starbucks) - calls to the US will be just as expensive from German prepaid cards.
If they want to make calls within Germany they should get a prepaid card for their own phone(s) if those are unlocked or if need be get a cheap Nokia or so. Germany has GSM (900 and 1800), UMTS (3G, 1920 to 1980 MHz and 2110 to 2170 MHz) and LTE (probably only in cities for now), so the phone needs to support at least the GSM frequencies.
Most prepaids in Germany piggyback on the network of a major carrier (who also have their own prepaids), so reception should not be an issue. However, they might be hard to get from outside the US, so the best bet is probably to buy the card in a shop at the airport when they land.