I did some research on the children's fitness tax credit as well. The finance department does its analysis, and the result was that not one more child was active because of it. That seems a little counterintuitive, but the report explains that the people who could already afford to put their children in sports were benefiting from the credit, and people who couldn't afford it still didn't have the cash, the outlay, to enrol their kids in sports.
To answer your more general question, I agree with any tax measures that will encourage children who otherwise would not be in sports to be involved in sports, and that doesn't necessarily have to be a federal government measure. A number of private sector non-profit organizations do just that.
Maybe there's a role for the federal government to play in encouraging that, by giving them the necessary tax benefits. I think of Canadian Tire, which is very active in my community in getting children to play hockey by providing free equipment to those who qualify.
I think anything the federal government can do to encourage children who otherwise would not be physically active needs to be considered.