The more we discuss anxiety and mental health, the more comfortable our children will feel asking questions. The more likely they are to understand what they are experiencing, when it occurs – and what to do about it.  

Reassure your child that everyone experiences anxiety at some point in their lives. Take the time to explain what anxiety is.

You could say something like:  "Anxiety is a natural feeling of worry or fear that happens when we sense a challenge or threat. It's part of being human and is actually meant to help keep us safe. There are ways we can learn to feel calmer and manage these feelings, like practicing relaxation techniques."

After an episode of intense anxiety, once they have calmed down, set aside time to talk it through. Listen actively and empathize with your child. Ask them how it feels for them and give them the time and space to respond in their own way. 

Ref https://www.childrens.com/health-wellness/5-ways-to-end-mental-health-stigma