What legitimate charity asks its volunteers to also canvas and raise funds? I would expect a legitimate charity to be happy to have volunteers, and to handle fundraising through a separate branch, staffed with people that are professional fundraisers.
My mom volunteers at Salvation Army - they don't ask her to beg for money from her friends. They don't even ask her to stand with a kettle and bell at Christmastime. If you want to volunteer to raise money, that's one thing. But if you volunteer to work with foster children, a charity should be happy to have the help, and not be asking you to also raise money.
I notice the original posting didn't even bother to name the charity (which would allow us to look it up, warts and all). Until proven otherwise, sounds like a scam to me.