What are the factors to be considered in custody claims between a parent and a third party?

[120] The factors to be considered in custody claims between a parent and a third party were considered in the seminal case of King v. Low and Low, [1985] 1 S.C.R. 87, 57 N.R. 17, [1985] N.W.T.R. 101, 58 A.R. 275, [1985] 3 W.W.R. 1, 16 D.L.R. (4th) 576, 44 R.F.L. (2d) 113, 1985 CanLII 59, [1985] S.C.J. No. 7, 1985 Cars­well­Alta 395, as follows: I would therefore hold that in the case at bar the dominant consideration to which all other considerations must remain subordinate must be the welfare of the child. This is not to say that the question of custody will be determined by weighing the economic circumstances of the contending parties. The matter will not be determined solely on the basis of the physical comfort and material advantages that may be available in the home of one contender or the other. The welfare of the child must be decided on a consideration of these and all other relevant factors, including the general psychological, spiritual and emotional welfare of the child. It must be the aim of the court, when resolving disputes between rival claimants for the custody of a child, to choose the course which will best provide for the healthy growth, development and education of the child so that he will be equipped to face the problems of life as a mature adult. Parental claims must not be lightly set aside, and they are entitled to serious consideration in reaching any conclusion. Where it is clear that the welfare of the child requires it, however, they must be set aside.