What is the evidential burden of demonstrating where the best interests of the child lie?

And finally, at p. 56: Until a material change in the circumstances of the child is demonstrated, the best interests of the child are rightly presumed to lie with the custodial parent. The finding of a material change effectively erases that presumption. . . . In short, the two-stage procedure required by the Divorce Act supports the view of Morden A.C.J.O. in Carter v. Brooks, supra, that once the applicant has discharged the burden of showing a material change in circumstances, “[b]oth parents should bear an evidential burden” of demonstrating where the best interests of the child lie (p.63).(emphasis added) The Extended Family