However, the elaborate treasure hunt, with 20 adults and children all dressed in their pirate best, ended with no treasure.
Someone else had got there first.
"When we got there, the treasure was gone. There were a lot of sad pirates," said Mrs Walker.
She created a Facebook group pleading with the public to return her son's treasure. "I was really gutted, because I had planned this for three years. I'll admit I did cry."
Four months after the party, which was in August last year, Mrs Walker received a text from her husband saying the police had found it.
The chest was so realistic that when a member of the public stumbled upon it they thought it was lost property and took it to the station.
Sergeant Drew Allison dedicated himself to discovering where the chest came from.
The chest, bought by Mrs Walker at a second-hand store, had "L. Ward" inscribed on the top. "I spoke with the (Whanganui Regional) Museum and Alexander Library to find where the chest came from, which led on to inquiries with relatives of Lillian Ward, who by chance were Mrs Walker's neighbours," said Mr Allison.
Said Mrs Walker: "The neighbours remembered we had a treasure hunt and the treasure went missing."