But on May 11, 2010, the couple had what he describes as a "mild argument", and what followed changed his life forever.
Later that afternoon, while he was at work, he says he received a text from his wife saying she was leaving, and taking their sons.
"By the time I got home, she was gone. She drove to Auckland, got on to a flight that night and flew out of the country, never to come back again."
His family had disappeared. The battle for custody has cost him $70,000, he says.
"It's heart-wrenching. It's completely exploded [my life]."
Last year 170 Kiwi children were abducted by their mum or dad into or out of New Zealand without the other parent's consent.
For the partners left behind, the fight for the return of their children - or just the right to have custody decisions made in their own country - can turn into a drawn-out battle with devastating emotional and financial impacts.
Each year, on average 100-120 parents apply under international treaty the Hague Convention to have a child aged under 16 returned for a court appearance in their home country to decide where they will live and who has custody rights. The convention aims to protect children from the harmful effects of their removal by ensuring they are quickly and safely returned home, but both countries must be signatories to invoke it.
More than 70 countries belong to the convention, including Australia, New Zealand, Britain and the US.
The treaty is guided by the principle that the home court is best placed to make custody decisions because they have access to all the relevant information.
But although the treaty can assist with the return of the child for court proceedings, it does not determine if they will stay there permanently.
About 70 per cent of Hague applications made by New Zealand parents involve Australia.
Internationally, about 2800 parents make return applications each year, including Cossey.
But after months without progress, the 41-year-old moved back to the US to remain in his sons' lives and agreed to have proceedings held under US jurisdiction. His New Zealand application has been closed.
"Thankfully I am able to legally reside in the US permanently, otherwise I would have lost all contact with my children by now," he says.
Each year about six parents whose children have been abducted seek help from Whangarei lawyer Rob Harte, who specialises in family law.
The abductions typically follow a relationship breakdown between an international couple where one parent wants to return to their home country with the children, he says.
In many cases the children travel to visit a parent and never return.
"There tends to be a winner and a loser and that's very tough. It becomes a very high-stakes matter so it's very upsetting to people."
Harte believes the Hague Convention works well to successfully help parents have a child returned home for court proceedings.
And he says New Zealand has the best system in the world for legal support, government support and turnaround time for court proceedings.
For parents who lose a child to a non-Hague country, a desperate situation can become almost impossible.
The number of children abducted to non-Hague countries is unknown.
They are at the mercy of the court where their children have been taken, and the chances of success are much lower, says Harte.
"You just about weep for those people, that's awful."
One such case is that of Northland woman Mihi Puriri, who has been locked in an eight-month custody battle with her estranged husband in his native Algeria over their daughters Iman, 5, and Assiya, 2, and son Zakaria, 1. The family travelled to Algeria in August because her husband Mohamed Azzaoui said his father was gravely ill.
But on arrival she discovered he was not, and her husband took the family passports, according to her London-based family spokesman.
Ms Puriri remains in the capital of Algiers fighting for custody of her children, who continue to be held captive in an apartment in his hometown of Mostaganem, says the spokesman. "All of a sudden the rug's pulled out from under your feet because the child just doesn't come back. If you couldn't deal with that in another country you're absolutely lost," says Harte.
For the children abducted, and their families, the effects can be devastating. "It's pretty dramatic on some kids - it depends on the child's resilience.
"You've got to assume that some of them are going to be pretty badly affected by it all," he says.
Days after the alleged abduction of his children, Cossey flew to the US and traced them to their grandparents' house near San Francisco, where they continue to live, with their mother.
It was a tearful first reunion, he says. "The kids didn't know what was going on. The entire event was pretty traumatic for them."
He now sees them two days a fortnight, under supervision. "It's disgusting. It's the most invasive experience that you can possibly imagine."
His estranged wife has also filed charges of domestic violence and sexual molestation of the boys against him - allegations he calls "ridiculous". His wife did not return calls from APNZ.
Other couples are able to reach a resolution, such as a case in which the safe return of a child was successfully negotiated between the parents and Harte without court involvement.
In another, an American mother took her child to the Netherlands and then to the US. After a year-long fight, the Kiwi father successfully had his child returned to New Zealand.
Harte advises parents to prevent the situation before it happened.
Parents living in separate countries and sending children on visits should put a consent order in place outlining that the child is on holiday and will be coming back.
If a relationship breaks down and a parent fears their child could be abducted they can get court orders put in place to alert Interpol if they try to leave the country.
Cossey believes the Government can do more to help parents whose children are abducted and believes anyone travelling overseas with their children should be required to at least present a letter of consent from the other parent.
He has filed a petition to the Superior Court of California for full custody of his sons, but holds little hope of the outcome he wants.
"My fear is that the legal system here revolves around status quo and they treat the kids as possessions that are currently in the mother's care.
"I think kids need both parents, and no matter what your relationship is there's only one thing that matters in the whole environment - and that is the kids."
High profile custody cases
* Kaikohe woman Mihi Puriri is locked in a custody battle with her estranged husband in his native Algeria over their three children. She claims he has been holding their two daughters and son captive in an apartment in Mostaganem since last August.
* Bruce Laybourn has spent hundreds of thousands of dollars and received ongoing consular help in trying to bring his son Dylan, 5, home after he was was illegally taken by his Turkish-born mother, Gulsen Nil Laybourn, in 2007.
* Northland mother Kay Skelton and her father Dick Headley were convicted over the five-month abduction of her son in 2006 as part of a bitter custody dispute with the boy's father. Both pleaded guilty and were sentenced to home detention.
