nag factor
n. The degree to which parents' purchasing decisions are based on being nagged by their children.
'Marketing and advertising to children has become a specialty unto itself,' agrees David Walsh, a psychologist and president of the National Institute on Media and the Family, a nonprofit group based in Minneapolis.

Trade conventions are held across the country to develop strategies to entice children to certain products and then get them to cajole their parents into buying the products. Those in the industry call it the 'nag factor' or 'pester power.'

Children between the ages of 12 and 17 typically will ask nine times for an advertised product in the hope their parents will give in, according to a recent survey conducted by The Center for a New American Dream, a consumer and environmental group based near Washington. More than half the parents surveyed said they do, ultimately, buy the product.
—Jenny Deam, “Targeting kid consumers,” The Denver Post, July 23, 2002
1984 (earliest)
Last February, using the background tale and motto, Tonka launched a TV ad campaign touting the GoBots, preying on what people in the industry call "the nag factor," the tendency for children, once alerted to a toy by advertising, to pester their parents about it. "Kids can be relentless," McDonald says, not disapprovingly.
—Carl Arrington & Irma Velasco, “Deck the halls with sqads of robots,” People, December 03, 1984