The Advertising Council and AT&T Thursday unveiled a major public service campaign to try to build support for local efforts to aid children. Council chairman Alex Kroll, chairman emeritus of Young & Rubicam, said the aim of the "Whose side are you on?" campaign is to turn the frustration of people over child-rearing problems to a useful purpose. "People are not so much apathetic as agonized; they have no clue what to do," he said. The campaign which was done by Y&R, is also supported by the Coalition for America's Children. The Benton Foundation is offering an 800 number and a Web site with information about local programs that can use help. The Ad Council hopes to get $200 million of free media on behalf of the campaign.