Recent experience at Boston with my daughter who is 3, has GE via NEXUS, and no fingerprints on file. The customs hall was quiet, so I approached the officer in the GE line and told him that in the past, my daughter's passport would scan and come up as not enrolled - no slip at all, X or otherwise - causing a problem when we had 3 pax and only 2 slips.

He took her passport and scanned it at the kiosk himself, and this time, she got the X slip. So something's changed on the software side, but it looks like children w/o biometrics might be able to use GE after all? At least at BOS. He told me next time to just use the kiosk normally for her, since that's all he did, and I should get the X slip. When he processed her at the X desk, he did confirm she was enrolled and had no biometrics (waiver). He did ask for her GE or NEXUS card, which we didn't have, but wasn't a problem since he believed me on the enrollment and was willing to look her up - but I would suggest bringing the card.

I do keep the blue slip anyway, just in case they ask for it. And I plan to keep doing that just in case the kiosk doesn't work.