Your post is difficult to understand because you seem to have answered your own question. Yes, under U.S. law, International passengers arriving from outside the United States or it's possessions must clear Customs and Immigration (C&I) at their first point of entry into the U.S. That does not mean first destination. It literally means first physical point of entry. If her International flight lands at IAD (there are no International arrivals into or departures from DCA that I know of which is usually the airport when you say arriving at DC), IAD is her first point of entry into the United States. Passengers on an International flight bound for DFW (Dallas-Ft. Worth) airport making an emergency landing at any other U.S. airport (say, Montgomery, AL) must clear C&I at that airport before continuing. It's a problem if that airport does not have C&I facilities and employees.
If you mean she is changing planes at IAD, yes she has to claim her luggage and clear C&I at IAD. What's the problem? Right ourside the C&I facility there is a place to recheck her bags to final destination in the U.S. If your child is an unaccompanied minor (15 years old or younger), an adult airport employee is supposed to meet her plane, walk her through the C&I procedures and escort her to her flight to Newark.
There are some countries (Canada is one) where the U.S. has C&I employees at the departure airport (say Ottawa) and those passengers clear U.S. C&I at the departure airport; so that when they arrive at their first point of entry (say DFW) they simply get off the airplane at DFW and proceed to their connecting flight's gate. Spain is not one of those countries.
