This incident happened in the land where most animals have the potential to kill you, Australia, more specifically with this incident Perth, Australia.
Kristy Edgelow, the mother of the children playing with the tennis ball, told Perth Now that her children were out playing near Swan River when they found the old tennis ball.
They played with the ball for around twenty minutes before one of the children spotted an oddity, there was something moving inside the tennis ball. They pried open the tennis ball to discover a strange tentacle crawl out of the seams of the ball, frightening the children. The octopus eventually made itself known by pushing itself completely out of the ball.
It was discovered that it was the deadly blue-ringed octopus which is one of the most poisonous marine animals in the world, the children had a very close call. The highly toxic octopus holds enough venom to kill 26 people. It's bite can cause nausea, paralysis which causes breathing difficulties and possible heart failure, resulting in death for the victim. Below, is an unrelated video of a blue-ringed octopus catching a crab.