Let's Chill Girl
COMO SE DICE
vamos a relajarnos chica !
there are two verbs you can use
tranquilizarse or relajarse
i would say "porque no nos tranquilizamos" or " porque no nos relajamos".
it's not quite what you want to say, which is more of a command/request but more of a request. i am sure she will get your intentions, whatever they may be.
advice please from some spanish speakers.