Progressive muscular relaxation is a great calming strategy, but can be very difficult to teach to kiddos. Luckily, Lori Lite has written a book called Angry Octopus, which makes muscular relaxation kid-friendly. As the title suggests, the book is about an angry octopus, who learns to cope with his anger when a mermaid teaches him how to engage in muscular relaxation. I’ve used this both in individual and group therapy with success! You can buy the book here.
There are also some youtube videos with someone reading the book aloud (just search “Angry Octopus”). The quality isn’t great (at least on the videos I have seen), but it’s an option.