Jordan Richards made the discovery after going to feed Rosy, Roxy, and Emmy at his home in Hedge End, Hampshire.
His mum Michelle said her son asked her if she had ever seen a guinea pig asleep before realising something was wrong.
She said she found three of their five pet guinea pigs dead, with six false widow spiders and cobwebs around the hutch.
The false widow is the most dangerous of the 12 species of biting spider known to be in Britain.
Michelle told her local paper: "Jordan told me they were asleep. He saw mummy upset and I told him what had happened.
"I am a childminder and we had children over - a lot of kids saw what happened.
"We don't want this happening to anyone else. We want to make as many people aware of this as possible.
"I am not scared of spiders and I've never been bothered about them until these ones.
"It's a rarity to be bitten and react badly but I have seen some horrific things happening to people recently.
"My husband Ray had to go out and get them. They had a look of agony on their faces. He's a tough guy but it was not good for him."
Michelle said her home backs onto a primary school and fears the spiders may have made their way onto its playing field.
However, insect expert Ian Burgess from the Medical Entomology Centre said it is unlikely the spiders killed the guinea pigs.
He said: "It could only have happened if the guinea pigs attacked the spiders and I cannot imagine a guinea pig attacking anything."