They looked after Kingston in the school holidays to give Ms Smith "a spell" but had not realised that she had gone to Northland.
"The first news we heard was that she was missing up north and we didn't know what that meant.
"And then the picture started to build about the flood," he said.
"We heard she'd possibly got caught in the flood.
"I got in touch with police up there and they were really good. They said she was missing ... [then] they rang me on Sunday morning to tell me they found her body.
"We actually brought Talia up. It's hard. It's really struck us hard."
On Saturday they told Kingston his mum was missing. Then on Sunday, with the help of Victim Support, they broke the news to him - though he might not yet have fully grasped the meaning of what had happened, Mr Smith said. "He sort of hung his lip a bit and wanted to cry. He made us all want to cry. He was silent for a bit," his great-grandfather said.
"I think being young, a 7-year-old, I think it hasn't sunk in yet."
The 72-year-old said he had got a couple of Kingston's cousins over to distract him with computer games and cartoons. "We don't want him being alone."
Ms Smith's aunt had travelled up to Auckland to formally identify her and take her home, Mr Smith said.
Her body was expected at Orongomai Marae in Upper Hutt last night and her funeral was scheduled to take place tomorrow at 11am.APN News & Media