Results from earlier studies show that women with type 1 diabetes are more likely to have overweight children. They are also less likely to breast feed or do so for a shorter period of time. Both aspects can affect the child's weight in later life.
Diabetic moms also need not worry about the composition of their breastmilk.
Researchers from University Hospital Groningen in Netherlands led by Christine M. van Beusekom found that the milk of patients with tightly controlled insulin dependent diabetes mellitus (IDDM) or the Type I diabetes, found no difference in milk triglycerides, lactose, protein, cholesterol, glucose, and no abnormalities in total fatty acids composition. (Source)
Diabetic mothers are thus encouraged to do their research and read up on the literature of the drugs they're taking. Then they should discuss their desire to breastfeed with their doctors so they can be supported or advised otherwise (in case breastfeeding is really not advisable).