This should help
http://travel.state.gov/visa/temp/without/....html#countriesCheck with your relatives if they have machine readable passports - You can tell by a thick central carboard page that says dont bend. In Australia, all passports issued after mid 2006 are automatically made this way. So I am guessing Gemany have introduced them as well to be on the visa waiver program.
I traveled to the US on such a system, and was no problem at all - I found the customers officers to be extremely helpful with issues that came up, and having a return ticket and departure date should seal the deal.
Maybe as an absolutely worst case senario, take the phone number of the nearest Geman consulate to your POE so they can help clear any silly miss understandings that occur. But as someone said, the customs guys are used to communication issues - so lots of pointing and smiles will get you a long way.
Finally - stress to your people not to get agitated or frustrated over the process, you will find the customs guys will respond to a good natured approach