Actually, when Ontario introduced their Ontario child benefit, they stopped recovering the national child benefit from social assistance. They overhauled their social assistance system to take the child benefit portion out and create an Ontario child benefit. People who receive the Ontario child benefit—and that's based on a test of income, so it's no longer tied to Ontario social assistance—don't see a clawback of the national child benefit supplement. That's as of last year, so it's a fairly recent development.
On April 10th, 2008. See this statement in context.