The site of Martijn, a group that says it lobbies for acceptance of consensual physical relationships between adults and children, also showed pictures of Crown Prince Willem-Alexander's 5-year-old nephew Lucas.
One photo of Princess Amalia was on the site until the Government Information Service ordered it removed Thursday night, said royal house spokesman Chris Breedveld.
Another reportedly was still on a members-only section of the site on Friday, Breedveld added.
Lawyers for the royal house have issued a summons ordering the Web site's controllers to remove pictures of the royal children or face a €50,000 (US$72,000) fine for each infringement.
The site's controllers must also sign a declaration acknowledging that what they did was unlawful, Breedveld said. If they refuse, the royal house will seek an injunction on Monday.
All the photos were believed to have been taken from the royal house's own Web site. The one that was removed from the public section of the Martijn site was taken by the crown prince.
"It is just a nice photo made by the prince of his daughter," Breedveld said. "We don't want it associated with this site."
Breedveld said the photos' use on the site amounted to a breach of both copyright and personal privacy.
A cell phone contact number for Martijn was not working Friday. On the Web site, the group said it was "not responsible for what members place on the forum and distance themselves from its contents."
Last year, a Dutch group campaigning for the legalization of pedophilia launched a political party that said it wanted to run in parliamentary elections. The bid foundered when the party failed to gather the 570 signatures of supporters necessary to allow it to register for the election.
The party's platform included lowering the age of sexual consent to 12.