The CBP officers at JFK (especially at T4) are very, very accustomed to processing passengers who do not understand any English at all. Sometimes for routine processing of VW-eligible citizens/nationals, they can process passengers with minimal conversation. For example, if they cannot get clear answers to their questions because of a language barrier, they might choose to inspect luggage instead. I've seen it work this way.

Of course, they can get interpreters on site if necessary. But in your case, I think if you just explain the process in advance to your family step-by-step, they will do fine.

If you have serious concerns and think your family might need assistance and/or interpreters, you should notify the operating carrier. As a last resort, you can contact the CBP office (the "ships office") in the appropriate terminal (based on operating carrier) and give the officers a heads up about the specific flight and arrival date.