I used to teach children whose internal state is generally easier to assess than adults.

I remember watching as one kid said to another "I believe in you" to help out in some moment of crisis. It struck me as odd - something cheesy they'd (probably) heard in a movie or cartoon.

But if they are saying that to each other then I could probably use it to communicate better with them as long as it was sincere.

I'd say it sparingly when I thought it might be appropriate or helpful. The reactions were surprising to me, not at the time I said it, but later on. I'm not suggesting I was a major influence that sets kids on the right track, but I think it helped out somewhat and for some reason that specific phrase seemed much more powerful than other forms of encouragement.