Awww, this warms my sad little heart.

Just be friendly around the family, and show them you care about their daughter. A little bit of very casual PDA around them is okay - I'm talking about hand holding, sitting close to her on the couch, etc. If the dad wants to talk to you about her, tell him you think she's amazing, and that you'll be extra careful not to hurt her since she's so young. Just do your best to come off as a mature, likable person, and you'll be fine. The fact that you weren't willing to lie to them is already something in your favor.

Good luck, and keep us updated. I love a good love story.