Melissa,
My daughter has CF and is now 21. She was always in the 5th percentile in weight and height but in proportion, so we never had a tube installed for additional feedings, even though our doctor thought about it. Sometimes now I wish we had because she topped out at 4'10 1/2". She doesn't seem to mind too much and I think she would have minded the feeding tube more, plus the risk of infection and all. Oh well, it's water under the bridge now. We had a hard time getting her to gain weight until she was in the 4th grade when we put her in gymnastics. She loved it and the added exercise really affected her metabolism and she put on considerable weight the year she started (pounds instead of ounces) She went from being a very picky eater to being a pretty big eater. She has been fortunate to have very little lung involvement and we wanted her to have as normal a life as possible for as long as possible. She is now in college and has been abroad several times and is looking at doing a whole semester of foreign study this next spring, so we are now in the stage of making sure we can get her meds to her while she is there. (Keep your fingers crossed that this all works out.) So far her lung function is still good, but her stomach gives her more trouble than it used to. The worst she has been through so far is a couple of nasal polyp surgeries and a blocked bile duct one year that was really rough going for a few weeks but treatable without surgery. Now they are starting to watch her blood sugar but it is not requiring treatment at this point. We live one day at a time and trust that our good Lord has a plan for her life that will bring Him glory. She is a very special young lady. I would not trade the experience of being her mother for anything in the world. My advice - find a physical activity that she likes and get her involved. (She also did cheerleading and volleyball in high school.)