Judge Geoff Rea told the court Crous had lifted up her cellphone to see how far away she was from her destination.
She took her eyes off the road for a few seconds, and when she looked up it was too late. She crashed at full speed into the rear of a stationary vehicle.
James Crous, 8, was wearing a seatbelt, but broke his neck and died at the scene.
Her 5-year-old daughter, and an 11-year-old girl in the other car, suffered cuts and bruises.
Her lawyer Scott Jefferson said the Crous family had moved to New Zealand from South Africa for a better life. He described Crous as a "doting parent" who homeschooled James.
Jefferson said the Serious Crash Investigation report highlighted that signs around the work site, which reduced traffic to one lane over the bridge, were a "little inadequate" given the 375m line of traffic on the northbound lane.
However "she ought to have seen it", he said.
Jefferson told Judge Rea that the crash has been devastating for the family and Crous is shattered by her own actions.
"The fact she has to live with her own actions is the ultimate penalty," he said.
Judge Rea told Crous that her sentencing was among the most severe and difficult.
"The result of your actions has been horrendous for you and your family."
He said the very reason it is illegal to use a cellphone while driving is because of exactly this. Paying attention to technology has led to a catastrophic result, he said.
In handing down a sentence of six months' supervision, Judge Rea told the emotional mother that "any penalty is insufficient compared to the life-long penalty of the loss of your son".
