The New York state Attorney General has announced settlements with three toy companies and one kids' broadcaster, after they were investigated for tracking children on their websites.
The settlements require Viacom, Mattel, and JumpStart to pay fines totalling $835,000 (£630,000) after a two-year investigation found they violated a US law protecting children online.
The 1998 Children's Online Privacy Protection Act prohibits the collection of children's personal information on websites for under-13s - information which could be used by marketers to target potential customers.
The fourth company involved, Hasbro, does not have to pay a fine because it was enrolled in a Federal Trade Commission-approved online-privacy program that had some problems, according to Attorney General Eric Schneiderman.
All four companies allowed tracking technology - such as cookies - on their websites, in direct violation of the law.
Schneiderman said: "The way the law is structured, the companies have the primary obligation to police their sites.
"When we notified them, they took immediate action."
Viacom received a penalty of $500,000, while Mattel paid $250,000, and JumpStart paid a penalty of $85,000.
Websites run by the companies include much-loved brands like Viacom's Nick Junior and Nickelodeon; Mattel's Barbie, Fisher-Price, and Hot Wheels; JumpStart's Neopets; and Hasbro's My Little Pony and Nerf.
A spokeswoman for Hasbro said it would be rolling out "a new, stricter online privacy protection policy for our partners", and will scan their sites "for any cookies, widgets or other applications that may violate our policy."
Mattel said it takes online privacy seriously, with spokesman Alex Clark saying when they are aware of a potential breach of the law, Mattel will "take prompt action to investigate and, if necessary, remedy the situation and look for additional controls to avoid a re-occurrence."
Viacom and JumpStart have not yet commented on the settlement.
"Now children live online and we have to police the internet as we seek to police our streets," Schneiderman said. "You track people so you can sell things to them.
"I don't want there to be a dossier on any child that can be used later to scam them."
The investigation in New York continues, as Schneiderman hopes other companies will heed the warning and stop using tracking technology on sites aimed at children.