"We went through the motions of Christmas, but it didn't fill the emptiness," he said.
Powell, 28, has been missing since Dec. 7. Her husband, Josh Powell, has told police he last saw her at 12:30 a.m. on that day as he left their suburban Salt Lake City home with the two boys for a camping trip.
Josh Powell failed to show up at work that day. Police consider him to be the sole person of interest in her disappearance.
Cox, of Puyallup, Wash., said the last few days have been very difficult. But his grandsons seem to be unaffected by their mother's disappearance and the national attention it has brought, he wrote.
"We have pictures of Susan and her family up, so I was a little concerned," Cox said. "But the boys just went on playing, so I feel they are doing well."
Josh Powell traveled with his sons to Puyallup a week ago to spend the holiday with family. Susan Powell grew up in the city south of Seattle and met her husband there.
There were no new major developments in the case Saturday, said Tom McLachlan, a police captain in West Valley City, the Utah town where the Powells live.