Assuming the website really violates COPPA, Google "COPPA violations" and grab some links to articles showing where the FTC sued over such violations and got big settlements. Then email those links to the boss (keeping copies of all this as others have suggested) and say something like "these guys got sued by the FTC and had to pay some big $, do you want to see our company get sued?"
If the boss takes an "I dont care" attitude or ignores the emails, go to the legal department or compliance officers with the same thing and say "I pushed this to my superiors and they chose to ignore it, I dont want to see our company held liable by the FTC, what should I do about it?"
If that doesn't work, consider packing up and leaving. Any company where the legal department doesn't care that the company is violating such a law and is one tip-off away from an FTC investigation (which could be a PR nightmare especially for a site that targets kids specifically) isn't a good company to work for.