OK, so we tried using GE twice with the kids....

1) The 8-year-old's prints scanned flawlessly both times, though the kiosk was unable to take his picture as he's obviously too short. No problem.

2) The 5-year-old's passport caused the kiosk to display the message that he was not registered for GE.

At YUL preclearance, the officer (whom you have to talk to in any case, and who does both immigration and customs) simply waved us through. At MIA, one officer made us fill in a customs form for the 5-year-old, and sent us to the diplomat line. The officer there looked over everything, requested the NEXUS card, and stated that he found it strange that the kiosk would refuse, as he thought all was in order.

He did confirm that getting an X on your printout does not mean automatic secondary, but merely that they have to see your passport to confirm your identity.

I suppose we'll be going back to a GE enrolment center to try to sort things out once and for all.

Overall, however, it was a good experience, and I'm glad things did work as they should.