'Custody' is a weasel-word. The parents are Responsible for everything about their children. 'Custody' is not a 'grant' or a 'privilege' given by the State, to be taken away at whim.
NO-ONE should interfere with parents and their children unless some criminal act has occurred.
Bloody busy-bodies !
First: it needs to be established that BOTH parents are Totally, Wholly, Individually, Jointly & Severally, Responsible and Accountable for the complete Financial, Mental, Emotional, Physical, Spiritual, Health, Education and Wellbeing of the children they produce together.
This means that Each is wholly Responsible; niether can prevent the other from the necessary participation and obligation. The failure or impediment of one puts the onus on the other. Neither can 'withdraw' or force the other away from the child.
It can be a matter of social 'organisation' or 'law', to determine the mechanisms of Accountability, but that accountability is to the Child, via a third-Party social instrumentality which is to 'aid' the child rather than dispossess the child of its own interests. In event of separation / divorce, BOTH parties MUST draw up a Plan to fulfill their obligations. Both must contribute, particulalry financially, which underpins most other provisions.
Secondly: it needs to be established that Interference with the obligations and rights of either or both Parents in relation to the Child is to be punishable , by Law'. This includes interference by one parent in the rights and obligations of the other parent.
Just what might constitute 'interference' might be determined in discussion but ought not encompass the normal day to day distinctive views of men and women, fathers and mothers in jointly deciding matters.
It would encompass outside individuals or 'agencies' who are currently geared toward busy-body, judgemental and often arbitrary punitive actions. For example there was a case last year of a child taken from her parents because a police 'raid' initiated by some animal welfare' snoopers acting on a complaint against them having a commercial Kennel business, accidently let the dogs out and used that as an excuse for the child being in danger. Such arbitrary action should carry a criminal charge.
Thirdly: Either or both of the parents may seek aid and assistance in fulfilling their obligations toward the child. This aid should have the agreement and approval of the other parent, where possible. Such 'aid' provided by State or non-State agencies must NOT subvert the rights and obligations of the parents. Such aid may be 'contingent' on specified factors and may carry contingencies in provision.
It must be sought. A third party might suggest it. No one can impose it without say, a Court Order. Aid provision may be limited so as not to 'replace' the parental obligation. It may carry an obligation to 'repay' in part, in some way. It may be 'time' limited. It would NOT reduce the obligation on a capable parent.