I'm a father to twins that aren't mine, and they are in high school. How do I tell them?

Many years ago, I had a best friend who was in the military. He got married to a friend of mine. Things were going well; they had a baby. During her pregnancy, I received a phone call from my friend telling me he was getting deployed. I remember the exact thing he told me: "Take care of her for me." There was already that trust, as I had introduced them, so we agreed. Best friend was gone. I took care of my friend/wife. Then, months later, she went into labor and had twins. I took PTO, and then weeks later, we received a knock on the door. From that, I've been supporting a family that isn't mine.

I don't mind, as that was the last favor my longest friend asked for, but the kids are almost 15 years old, and I really don't know how to put it. It's been far too long of a secret, and I honestly am scared of the trust, image, and attitude changing towards me. What do I say, and how do I approach them?