This is a paid for biofeedback app that uses a heart rate monitor and a tablet style game, so kids can learn to calm themselves down. It is $40 a month, but if you are at the stage where biofeedback could be useful and willing to practice it could be worthwhile.
Feeling identification is a huge part of the work with kids. I use a variety of tools to help kids identify and understand their feelings in themselves and in others. Kimochis offers feeling pillows, feeling posters, feeling cards, characters, and stories related to common emotional issues in small children.
If you are trying to introduce your kids to concepts like mindfulness or tools like calming sleep stories or breathing an subscription based app like Calm Kids may be the way to go.
Feelings in a Flash - Emotions Flashcards
We love this set of emotional situation and emotions with coping skills flashcards once kids have a basic understanding of their emotions.