"Let's pretend we're blowing up a giant balloon. We'll take a deep breath and blow it up to the count of 5."
If you tell a child to take a deep breath in the middle of a panic attack, chances are you'll hear, "I CAN'T!" Instead, make it a game. Pretend to blow up a balloon, making funny noises in the process. Taking three deep breaths and blowing them out will actually reverse the stress response in the body and may even get you a few giggles in the process.