Can't say, "No negative talk" as that itself is negative.

Drill this broad concept into the team at a meeting. Get a prompt or clue to signal to the other person such is unfolding and its displeasure. 

Then the next chapter in that lesson is how to, "Fake it to you make it." No matter what, "Look good." That's a step. Sound good. Eventually, the power of positive attitudes take over the team and the behaviors of one's life. Lay the shovel down and begin today.

As humans, we are called to be realistic. But as young people, we've got the charge to be anything we want to be -- with work, and with time. The positive journey, the positive life, the happy outlook is a choice. Embrace it.

Attitude establishes altitude.