According to www.parents.com “The phrase "calm down" is controlling, dismissive, and suggests a person's feelings are invalid. It can also exacerbate anxiety. I know when someone tells me to "calm down" when I feel upset, my mind races even more.”
Source: Saying ‘Calm Down’ to Anyone Doesn’t Work, Especially Your Kid
This is all well and good, but when a child is in the throws of a meltdown what else can you say? The information attached suggests some other phrases and actions you could use to try and help your child process and manage their feelings.