The fire, in the early hours of Wednesday, totally destroyed the hutch in the back garden of the Sherwood family home in St Budeaux, Plymouth.
The one-year-old female pig was found dead, but the remains of the three-year-old male pig have not been found.
Amy Sherwood said her son Cody Buckley's heart had been broken.
"When I told my little boy what happened, he said it was the worst day of his life and asked why anyone would do that to him," Ms Sherwood told BBC News.
"We're totally devastated - I feel numb, upset and angry."
Ms Sherwood said she was very grateful to her next door neighbour who saw the fire in the back garden and raised the alarm.
She said CCTV footage belonging to another neighbour had been viewed by police and appeared to show a lighter being ignited.
"It's sick and evil... if they can do that to a child's pets, what else are they capable of doing," she added.
Devon and Cornwall Police, which is investigating the attack said the "horrific crime" had caused the family a great deal of distress.
"There is absolutely no excuse for animal cruelty, and we are working hard to identify those responsible," PCSO Aimee Cattell said.
Two guinea pigs are thought to have been killed in a "sick and evil" arson attack on a five-year-old boy's pet hutch.