The contest, set up by the charity Grief Encounter, is being held in honour of Holly Wells, who was murdered in Soham in 2002 with her school friend Jessica Chapman.
Holly's father, Kevin Wells, will be on the panel, joined by poet John Hegley, actress Dani Harmer (from The Story of Tracy Beaker), author Rosie Thomas and children who have lost a loved one.
They will be looking for thought-provoking and touching poems from 7- to 11-year-olds and 12- to 18-year-olds. Winners will be published in an anthology and stand to win a pound;1,000 cash prize for their school, plus vouchers.
Mr Motion, who lost his mother when he was a child, said the competition would "demonstrate the power of poetry to hearten and console us, while recognising excellence in writing".
"I'm delighted to be judging the competition and wish it every success," he said.
The Grief Encounter project was founded in 2003 by Shelley Gilbert, who lost both parents by nine years old and set up the charity to help others in the same position.
The closing date for competition entries is June 28.