Jellyfish tentacles release a very irritating toxin that leaves a line of raised, itchy, painful bumps. If your child is stung, rinse the affected area with saltwater (or vinegar, if you or the lifeguard have it on hand). Don't use fresh water, which can worsen the injury. If stingers are left on the skin, apply a baking soda paste and "shave" the area with the edge of a credit card to remove them, which will slow the injury's progress. If an arm or leg was stung, immobilize it with a splint to stop the venom from spreading.