Sesame Street's YouTube channel has been taken offline after hackers uploaded several pornographic videos.
The changes appeared to have been made by someone called Mredxwx, although a member who uses that name denied involvement.
A message posted alongside the adult videos stated: "Who doesn't like porn kids?"
It is understood that the Sesame Street account was taken offline within 20 minutes of the problem being spotted.
In a statement, Google - which owns YouTube - said: "YouTube's Community Guidelines prohibit graphic content. As always, we remove inappropriate material as soon as we are made aware of it."
The American children's show has more than 140,000 subscribers and its videos have been watched almost half a billion times.
Previous porn attacks
Security blogger Graham Cluley grabbed several screenshots of the offending content before it was removed.
It included a message which read: "Im Mredxwx and my partner Mrsuicider91 are here to bring you many nice content! Please don't let Sesame Street to get this account back kids."
On that user's registered account, Mredxwxx posted a short video clip stating: "I did not hack Sesame Street. I am an honest youtuber. I work hard to make quality gameplay videos and most important I respect the community guidelines."
It is not the first time that areas of YouTube intended for young people have been found to contain pornographic material.
In May 2010 the site removed hundreds of videos which mixed adult material with footage of children's TV programmes and music videos.
The Jonas Brothers and Hannah Montana were among those whose videos were replaced.