The occurence of genetically inherited diseases are in most cases, not 100%. There is always a chance that they may or may not manifest. Part of this depends on lifestyle and environmental reasons as well. Some diseases like diabetes and heart disease can be avoided or delayed by lifestyle modifications.
Genetics can be quite complex - two siblings who are not identical twins, may not experience the same type of diseases or medical problems.
That being said, kamma depends on one's intentions. If you had children without evil intentions, then it's not bad kamma.
Whether your children would get a specific disease or not, cannot be known with perfect certainty. However, with perfect certainty, we can say that they will grow old, experience disease and death. They will also experience pain and pleasure. They will also burn with the fires of passion, aversion and delusion, for as long as they are unenlightened.
The fact that your child would experience sufferings of any kind, is not caused by their birth. Birth is merely a symptom of suffering. It's not the cause of suffering.
You should not feel responsible for the phenomena of samsara. It moves along perfectly fine on its own.