I don't want to alarm you, but make you aware of the situation that we were placed in with a heart murmur. Our cat, Andy, was diagnosed with a murmur as a kitten and the vet said that this was common, and probably would not be a problem. At about 15 months Andy started having difficulty breathing when he was put under stress. It appeared that he was having an asthma attack so I called the vet. The first time he was diagnosed with having a vaccine reaction as it was the day he was vaccinated. They treated him, and sent him home. A week later it happened again and I called the vet. He said keep him call for the night and bring him in first thing in the morning for x-rays. The x-rays revealed that he had a pectus excavatum, otherwise known as a deformation of the sternum that was putting stress on the heart. In order to treat this we would have to see a veterinary cardiologist at a state university. We took him to Michigan State CVM and saw a cardiologist and within 5 minutes he said it sounds like he had heart disease and they took him for an echocardiogram. The echo showed that his heart had a tissue membrane bisecting the upper right chamber of his heart and the Dr. said he wouldn't be able to live more than another month with this condition. The only option for Andy was an experimental surgery that had only been attempted 3 times before and not one of the 3 other animals survived the surgery. We sat in that examination room, while Andy lay in an oxygen tank after getting stressed out from having the echocardiogram and having an attack, trying to decide what to do. After 6 hours of sitting in the exam room we decided to do the surgery. The Dr. said he would not charge us for the surgery and he wanted permission to try a new technique that they hadn't attempted before. Andy went into surgery the following morning, and 3 hours later the phone rang. The surgery was a success, they were able to get his heart beating again, but it took them too long and Andy didn't wake up.
Andy went from being a perfectly normal, happy cat with a simple heart murmur to dying within 5 weeks. I can whole heartedly understand why you would keep those kittens, but please take them to the vet frequently (every 6 months at least, as opposed to annually), and don't think that you are overreacting for calling the vet for a slight wheeze or something. I hope this is not your experience, but I felt that too many people were telling you that it is probably nothing to worry about.