When parents separate, where the children live, how much time they spend with mum or dad, can be hard to agree.
Sometimes a child starts refusing to see their other parent.
This autumn, social workers who look after a child's interests in the family courts are being given new guidelines to help with these cases.
For the first time this will consider the possibility a child has been deliberately turned against one parent, by the other.
Parental alienation, as it's called, will be just one of the options a social worker might consider.
It's a controversial concept which the courts have been trying to grapple with for years in cases where the parents are locked into entrenched legal action over contact.
There is no consensus and not a great deal of research, so how might it be considered by courts here?
The intensity or frequency of behaviour might be one of the ways this is set apart from the disagreements that are often part of separation.
"Think of a child experiencing a separation, the mother or father bad mouthing, or withholding warmth and affection unless they agree with an argument," says Sarah Parsons of the Children and Families Court Advisory Service.
"If it's repeated it can have an invasive, intrusive effect on wellbeing. A child can think the only way to stay safe is to side with one parent and reject the other."
The recognition in the advice to social workers does not mean other possibilities won't be considered.
Children might refuse to see a parent because of authoritarian or abusive behaviour, or because they feel naturally emotionally very attached to one parent.
High level of conflict
So how common might parental alienation be?
Around 10% of separated parents have contact agreements made through the courts.
Each year around 1,400 cases return to court in England seeking enforcement of contact orders, according to research published by the Nuffield Foundation in 2013.
Professor Liz Trinder, from the University of Exeter, has studied cases where there is a high level of conflict between the parents.
She thinks the idea of parental alienation as a pattern of behaviours needs to be treated carefully, because the courts have a duty to consider the child's best interests.
Part of that is listening carefully to what children say themselves about their feelings.
"The problem with the alienation concept is that if your premise is the child has been brainwashed, it means you can't trust what the child is saying to the court. So if you make an accusation of alienation it almost automatically casts suspicion on anything the child might say."
Professor Trinder believes England could learn from Canada, where concerns emerged of counsellors and expert advisors to the courts whose careers were invested in a strong belief in parental alienation.
The consequences of a diagnosis of alienation can have a huge impact on a child, she argues, with some supporters of the concept arguing it should lead to a transfer of care and residency.
"Supporters of alienation will generally insist that should be done literally overnight. So the child is removed from the alienating parent and placed with the so called innocent parent, and the child won't have any contact with the first parent. For me that feels like child abuse"
The charity Women's Aid has also expressed concern this year that accusations of parental alienation may lead to a history of an abusive relationship not being given enough weight.
While some campaigners have tried to press for its recognition as a syndrome the charity warned the development of the theory in the US has been highly disputed.
All of this is very challenging for the family courts to manage.
In one case in 2010 which provoked debate a family court ordered a 12-year-old boy should be transferred from living with his mum, to his dad.
Contact with his father had broken down four years previously, but the court battles had lasted most of his life.
An appeal by his mother was turned down.
Then an appeal on behalf of the boy led to him being put into foster care with the aim of building contact with his father before a move.
In contact sessions with his father, the boy was described as sitting with his head in his lap, hands over his ears.
The court eventually accepted the order could not be enforced.
The judge was told the father, described as "wholly deserving", was heard sobbing as he left the final contact meeting.
CAFCASS said it hopes its guidelines will help form a basis for social workers to explore what is happening with both parents, rather than attributing blame.
But Sarah Parsons also said it is important not to overlook the lifelong impact where a child is alienated.
"It can be devastating, an adult can look back on childhood and feel misled by trusted parents, adults can have mental health issues, just like any other abuse would.