“Be on our side,” the clutter-free advertisement reads. “We are the side of peace and justice.” It shows two men smiling. One is Palestinian, the other is Israeli, and each is accompanied by a smiling young girl. The ad, which first appeared in three Bay Area Rapid Transit (BART) stations on Dec. 5, is not in the least bit edgy — until you get to the tag line: “End U.S. military aid to Israel.” Paid for by Northern California Friends of Sabeel, American Muslims for Palestine and Jewish Voice for Peace, the ads made an earlier run on the platforms of the Chicago Transit Authority in October 2010, and they represent a new, cuddlier look for a familiar message. “Visually, it felt like ads that you see for children’s hospitals,” Matthew G. Jarvis, assistant professor of political science at California State University, Fullerton, wrote in an e-mail after seeing the ad. Jarvis, who studies political behavior and public opinion, felt that the jump from families, peace and justice to the end of U.S. military aid to Israel was too abrupt. “It’s happy, then wrenching,” he wrote.