I had to go through that same system recently, and noticed that families with children - in the same, nornal boarding group - were all called to a line to the side of the face scanners that had the normal boarding pass scan system. Maybe you have to ask to be in that line? It was very confusing in the moment because a sign implied that line was for the 'premium' members (or whatever the preboarding group is called) and the face scanners were for regular boarding... (+ zero information about opting out)