one more thing, start with a Canadian attorney..

I believe that the amount of child support will be calculated according to the governing rule in your dwelling location....it's likely your canadian attorney will have U.S. attorneys they regularly work with ...they will take care of the business in U.S. side...

One last thing, check your potential attorney's credential before the hiring, make sure that he is Canada or U.S. educated (i.e. law school in U.S. or Canada), and go with a reputable law firm...