We got Nexus and Global Entry in November 2011, and after they allowed GE for children under 14, I enrolled my kids several months ago.
On our return on FCO-FRA-SEA last week, the customs officers were surprised that all four of us had GE. They had never seen a family with young children who had GE.
It did make a big difference. We would have had a 30 minute wait in immigration and a long wait at customs, as two DL flights from AMS and CDG came in just a few minutes before we did.
^
It is worthwhile domestically when they can get three beeps for PreCheck (I know you can take one child under 13 through PreCheck with you, but it's nice if they can do it on their own)