Emotions are continuously popping up in response to the events around us. That doesn't mean that they define who we are, or our value!
It's often the way we interpret these emotions that cause us distress. If your child doesn't get invited to a party they may feel sad and isolated, and interpret the feeling and event as "no one likes me, I'm unlikeable". When we look objectively at the event, there may be many reasons why they were not invited. There's also the fact that if one person doesn't like us, it doesn't mean no one likes us. The feeling of sadness is valid, but the interpretation may be wrong or unhelpful.
Here is an easy process to teach children how to reframe negative thoughts and beliefs. This one definitely takes practice, but the more we try the better we get at it.
Take notice of what sensations have arisen. How does your body feel? What emotions has it brought up? What kind of thoughts have arrived?
The important thing is not to make any judgements, just observe.
Name the objective facts happening outside of personal judgement. For example, instead of "no one likes me", say "I keep having the thought that no one like me".
Acknowledge when something happens in the moment, we don't want to ignore how we feel. But remember that it's a feeling, it's a thought, it's a sensation. Observe it, describe it, then accept it came up, and move forward.