Todd Haynes has been in the zone for quite some time now, creating a remarkable streak of films that establish glorious illusions, and then burrow deeper for the real, messy deal. Wonderstruck, the director’s first movie for a younger audience, feels like an anomaly in other, less intriguing ways—including an atypically slack narrative and an occasional case of the cutes. But then the third act kicks in, and everything gets terrific.