Unfortunately only a few nutritional studies have been done on diabetic dogs and the findings have generally been inconclusive. While the general consensus a few years ago was to feed a high fibre diet, it's no longer clear whether that is the best approach or not. Indeed, responses seem to vary from dog to dog so recommending a suitable food is very tricky indeed.
In general these days, nutritionists and vets tend to simply recommend feeding a healthy diet that the dog likes eating. If your dogs have been otherwise well on whatever they are eating now, then there is probably no need to change.
Consistency is key as any sudden dietary changes might lead to blood sugar level spikes or crashes. Try to keep the feeding amounts and times the same each day. 2 meals per day, 12 hours apart is usually best but some dogs will also need a smaller snack or treat between meals to prevent the blood sugar from falling too low.
Managing canine diabetes is an extremely complicated process and should be done in close consultation with your vet.
You can find much more information here: http://www.whole-dog-journal.com/issues/15_5/features/Canine-Diabetes-Diagnosis-and-Treatment_20521-1.html