A traffic light key ring which feature 3 emotions - happy/ok/sad, or happy/not ok/ angry. (colour of keyring may vary).
 
Depending on how you want to use it. This can be very useful for anybody who struggles with expressing their feelings leading to angry outbursts and meltdowns. As it is on a key ring the person using it can indicate if they are feeling good/happy (Green) if they are not great/starting to struggle (amber) really upset/angry/struggling/need to return home or leave situation. Could be used in a classroom environment to indicate when a break is needed, when in public places or busy environments such as shopping etc.
This can also be used by an adult to show their child if their behaviour is good (green) amber (need to think about it) red (not acceptable/stop). This may be useful when out and about to help a child gage their behaviour. Also useful for children who respond better to visuals than to language.