The Somerville News article describes the policy as "children under 12 are not permitted to enter the theatre past 6 p.m." This is not the same thing as your summary, "banning children under 12 after 6 pm." They're not forcing kids out of the theater at 6; they're prohibiting them from entering shows that start after 6 — so effectively it's a ban on under-12s being in the theater after 7:30 or 8, which does not strike me as unreasonable at all.