This works extremely well for us...
Petrodex Finger Toothbrush Gloves - Health Care - Cat - PetSmart
I got this from PetSmart. I cut the whole index finger off of the glove and place it on my finger. It is very thin and perfect for tiny mouths. The red rubber ones were too big for Prince's mouth, and regular dog toothbrushes were too big and awkward. I wish they sold only the index finger, but I don't think they do. You can wash and reuse the "finger" as well, like a toothbrush.