What is the test for retroactive child support?

A fundamental tenet of the modern law of child maintenance is that the right to maintenance is a right belonging to the child, not the parent. This simple statement has profound implications for the issue of retroactive support. As the right belongs to the child it cannot be waived or bargained away by the custodial parent or lost due to that parent's neglect, delay, or lack of diligence in enforcing the right. As Wilson J. stated in Richardson v. Richardson, 1987 CanLII 58 (SCC), [1987] 1 S.C.R. 857 at 869-70: The legal basis of child maintenance is the parents' mutual obligation to support their children according to their need. That obligation should be borne by the parents in proportion to their respective incomes and ability to pay.... Child maintenance, like access, is the right of the child.... For this reason, a spouse cannot barter away his or her child's right to support in a settlement agreement. The court is always free to intervene and determine the appropriate level of support for the child.... Further, because it is the child's right, the fact that child support will indirectly benefit the spouse cannot decrease the quantum awarded to the child.