A council is to ask all public bodies it deals with not to "discriminate" against parents who do not live with their children.
Cardiff council will now write to all other local authorities in Wales asking them to adopt the same principle.
Non-resident parents claim schools, hospitals and GPs' surgeries will often give information only to parents living with children
The number of single-parent households has risen yearly in Wales since 1991.
In 2007, the Welsh Assembly Government issued guidance to schools on how to deal with separated or divorced parents.
Cardiff councillors have voted to remind all public bodies and partner agencies that the local authority deals with that both parents are equally important and should be given access to the relevant information about their children.
Parents who have moved out of the family home - both women and men - have told BBC Radio Wales that previously healthy relationships with their child's school or GP surgery can quickly turn sour.
John, from Cardiff, a non-resident father, who spoke anonymously to Good Morning Wales, said: "Schools, doctors and even dentists were then saying all the information then can only go to the parent where the children are living."
Two fathers whose children both had fits in school said they were not informed of the incidents because they did not live with the child.
One faced further problems when he turned up at the hospital - with documents to prove who he was - but was refused information about how his child was doing and could not see the youngster.
A mother, Jo, [not her real name] said her son's secondary school refused to interact with her despite a court order giving her joint residency of her son, even though he lives most of the year with his father.
She said: "I need to know, from the school, how he's doing.
"It makes it difficult for me to deal with my own child."
Organisations which help non-resident parents say a presumption of "shared parenting" in the event of separation or divorce is the way forward rather than assuming that only the parent with whom a child lives has full responsibility.
David Evans, Welsh secretary of the National Union of Teachers, said schools had to put the rights of the child ahead of the rights of a parent.
He urged Cardiff council to issue "clear guidance" to schools.
He said: "Schools put in place procedures which will resolve any issues or complaints that might come forward, in a very sensible way."