Security forum reports suggest that the channel was taken down within 20 minutes of the attack starting.
According to the Reuters and BBC newswires, a hacker by the name of Mr Edxwx uploaded several pornographic videos some time on Sunday, as well as a message stating: “Who doesn't like porn kids?”
In a statement, Google, the operator of YouTube said: "YouTube's Community Guidelines prohibit graphic content. As always, we remove inappropriate material as soon as we are made aware of it."
Infosecurity notes that the YouTube channel has approaching 150,000 subscribers and the videos have been watched around 500 million times.
Sophos senior security consultant Graham Cluley says that he managed to grab several shots of the offending screens before they were removed.
Interestingly, he reports that the user Mr Edxwxx later claimed that s/he did not hack the channel, noting that s/he is an honest youtuber and stating: “I work hard to make quality gameplay videos and most important I respect the community guidelines."
Cluley says that, as well as uploading pornographic videos, the hackers also changed the Sesame Street channel profile on YouTube:
WHO DOESN'T LOVE PORN KIDS? RIGHT! EVERYONE LOVES IT! 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 :( PLEASE...LET ME AND MRSUICIDER91 HAVE IT AND WE GONNA MAKE ALL THE AMERICA HAPPY!
"The truth of the matter is that the channel is regularly visited by young children, and parents trust that the page will be safe for them to view", said Cluley.
"Attacks like this prove that websites with particularly vulnerable audiences need to be monitored regularly, and protected with the highest possible security controls. In addition, parents need to be extra vigilant about keeping an eye on the websites their children are visiting – and remember that even the seemingly most innocent websites can be compromised."