She described Finn as a sensitive boy interested in everything, with a great sense of humor. "Charlotte had gigantic blue eyes which made you melt," she said.
Mortimer's attorney also read a statement from his parents, who spoke of their lost grandchildren and said their son was plagued by depression.
"We know that our son Thomas has done something horrible, but he has been the nicest, most compassionate person we have ever known," they said in the statement. "No one who's ever known him can imagine him doing what he's done.
"We didn't think ... until the events that he was so depressed," they said. "We wish we could have done more for him and his family."
Mortimer did not make a statement in court, where he remained handcuffed and shackled throughout. He simply confirmed to the judge that he discussed with his attorney the consequences of changing his plea and he was willingly taking responsibility for his actions.
But he did try to explain himself in the message he left for his family and friends at the crime scene before he tried to end his life before fleeing the home.
Mortimer said he should have written about living with Laura and her family all these years, instead of "bottling up my anger, frustration, resentment and hatred, and let it fester until one murderous night," prosecutors said in court on Wednesday.
"Ultimately, I did these horrible things because I could not cope with the responsibilities I took upon myself. I was too cavalier with life, especially others' lives," he said. "I do have remorse with what I have done. I wonder what life would be like if I did not chicken out."
He seemingly tried to justify killing his young son and daughter, suggesting they were better off without him and their mother.
"I think of the future and think of Finn or Charlotte being teased or bullied and my heart breaks. I cannot think of a more positive situation. What have I done? I hate myself more than ever," he said.
Rodrique Ngowi can be reached at www.twitter.com/ngowi