Sonny Bill Williams has "let a lot of people down" as his decision to move to French rugby union leaves a children's charity without a major drawcard for its biggest fund-raising event yet.

"It's very disappointing he's pulled the plug three weeks before hand," said Fran Dobbie, CEO of Essere (To Be) Living Skills, a charity that delivers educational workshops to prevent anti-social behaviour, depression, bullying and youth suicide.

The 2008 Essere Ball is due to be held in Pyrmont on August 19, and Williams was scheduled to make a guest appearance to talk with Karl Stefanovic about the charity's major focuses, and his career.

"He was going to talk about the importance of prevention, the importance of dealing with bullying and prevention," Ms Dobbie said.

While other guests at the ball include NSW Premier Morris Iemma, Treasurer Michael Costa, Mario Fenech, personality Bessie Bardot, Bulldogs chairman George Peponis, blogger Samantha Brett and journalist Jeff McMullen, the Essere website boasts Williams as the main attraction, calling it an evening with Sonny Bill Williams and friends.

As the charity is only half-way to filling its 600 person capacity, Ms Dobbie hopes others stars will step up and replace Williams so that the night can be equally, if not more, successful.

Ms Dobbie also said Williams had given the charity an undertaking about three weeks ago that he would be involved in other workshops and support the charity, perhaps as an ambassador.

"For a lot of young people that we educate and deliver our work to he was a role model. A young man who [showed] leadership, teamwork, etc, he was actually like a drawcard for this event ... It's very inspiring for young people because when a young footballer talks about how they control their anger, how they control situations of bullying, it's very powerful. The young people sit up and listen."

Although children will not be attending the event, Ms Dobbie says they are aware that Williams has pulled out.

"It's very disappointing for us as a charity but also for the children. The children know that this event was important because it brings awareness to what we do."

Ms Dobbie thinks it will alter the children's impression of Williams as a role model. "[But] I hope that he will be a man of integrity and turn this into a positive ... I don't think he's doing it with the intention of hurting anyone. I just think he has his own issues and that's why he's made the choice, but he has let a lot of people down."