A broken leg is very painful. But most animals hide their pain well as a defense mechanism. Too often guinea pigs will jump out of children's arms and land on the floor, injuring a leg. Simple fractures are not visible through the skin, but if your guinea pig is limping, I would suspect a break. With compound fractures, bones often protrude through the skin or a bruise (hematoma) is visible. Take your guinea pig to the vet immediately for x-rays and treatment so that the injury doesn't worsen. When children play with guinea pigs or any small animal, they should be taught to sit on the floor and handle gently.That way there's less danger of injury to the animal.
