But when she visited the park last Friday she was upset to find only an empty space where the seat had been.
"This isn't something that could have happened easily.
"Someone has to have seen this seat somewhere or been part of its removal," she said.
"This is a sad piece of theft, a very personal item which is priceless to us as a memory of this young man and something we chose to share with our community so that they could take a rest and enjoy a view, something our grandson cannot do."
She urged the thieves to return the seat and anyone who knew where it was to call Kawakawa police on (09) 404 3150.
Anonymous tips can be passed to Crimestoppers on 0800 555 111.
The park in Callaghan Rd with impressive views of the Maromaku and Motatau Valleys has been set up by volunteers.