Here is what I would impress upon the young lovers.
First, I would explain the basics of sex... yes, rolling teenage eyes are included. Explain that even the first time can result in pregnancy. Explain that, for all they know, she is pregnant right NOW.
Explain that even if they were smart enough to use a condom, they probably used it wrong.
And yes, they won't like talking about this stuff with their parents. Good. They want to play with adult toys then they can be talked to as adults.
Then, proceed on the basis that she is. In the presence of the girl and HER parents, walk them through a supermarket. Make sure they understand the prices of diapers, formula, incidentals and babysitting.
Make a point that the grandparents are not paying for a dime.
Then, call up some statistics about the cost of child birth, prenatal care and child care. Get some local quotes on child care facilities and ask who is paying the $150 a week while they are in school.
Next up with statistics that show the number of teen mothers that complete high school, college and post graduate work. The number is terribly low... even today.
Finish up with the statement, "Now then, say goodbye. You two cannot be trusted together and, therefore, are no longer allowed to see one another. Your sexual escapade cost you your relationship... even if that means I have to walk to the door of your school in the morning and walk you out in the afternoon. Even if it means the only cell phone you can get is the one that only calls your father and me. Even it if means I monitor your text messages and emails on a nightly basis.
And no, if the little girl IS pregnant, the boy will have the right, after paternity is established, to see the child ... not the MOTHER... until her parents allow it. So say goodbye to each other. If you are determined to screw up your lives, I will do everything I can to stop you."
Tell me how it works out.