HI jennie, I hope you have been welcomed already, I can see you are fairly new to the site

Ten clama is the correct choice.

You can say "Que tengas calma". But this form is used when you want to emphasise :

I told you to be...

Te dije que tengas calma.

"Ten calma" is in the imperative mood, so it is a command. "Tenga calma" is in the subjuctive mood, so it is a recommendation.

The first is the subjunctive form of "tener", which means that your are expressing a wish, desire, hope, etc. "Tengas" is the familiar 2nd person present tense. In your example, you might say "Quiero que tengas calma." "Quiero que" can be implied. "I hope that you calm down/be calm".

The use us "ten" is the imperative form - more of a command or directive. Also 2nd person. "¡Ten calma!" - "Be calm!" or "Calm down!"

Click on the "More" button, then "conjugation". Type in "tener" to see all of the forms of this verb.

A common way to tell someone to calm down is !Tranquila, tranquila¡

Ok, so I am thinking that it depends on the situation? Like, if I'm talking more from my point of view- talking about what I hope for or want someone to do- I would say "Quiero que tengas calma." This would have made sense in the situation because it was not urgent that my son calm down. However, if my son had been in hysterics or about get into trouble or something, a better choice would be "Ten calma" with exclamations maybe? Or, maybe I am understanding this incorrectly still, and I should generally just say "ten calma."