A coalition of nearly 20 children’s advocacy, health and public interest groups plans to file complaints with the Federal Trade Commission on Wednesday, asserting that some online marketing to children by McDonald’s and four other well-known companies violates a federal law protecting children’s privacy.

The law, the Children’s Online Privacy Protection Act, requires Web site operators to obtain verifiable consent from parents before collecting personal information about children under age 13. But, in complaints to the F.T.C., the coalition says six popular Web sites aimed at children have violated that law by encouraging children who play brand-related games or engage in other activities to provide friends’ e-mail addresses — without seeking prior parental consent.

At least one company, however, said the accusation mischaracterized its practices, adding that the law allows an exception for one-time use of a friend’s e-mail address. As of late Tuesday, the companies said they had not received copies of the complaints. Obtaining information about adults’ social networks to e-mail marketing messages to their friends is a common industry practice called “tell a friend” or “refer a friend.” But now an increasing number of children’s sites are using the technique by inviting children to make customized videos promoting certain products, for example, and then sending them to friends.

The sites cited by the advocacy groups include McDonald’s HappyMeal.com; Nick.com, the Nickelodeon site owned by Viacom; General Mills’ ReesesPuffs.com; SubwayKids.com; another General Mills site, TrixWorld.com; and Turner’s CartoonNetwork.com.