S.L.
When my kids started brushing their teeth they did the same thing, they usually sucked off the paste, and so I let them do the front teeth and when they were finished I brushed the back teeth so they didn't stay un brushed. Maybe I was doing too much for them, but out of 8 kids none, NONE, ever had cavities until in their 30's and only two so far have had them. I doubt it was from me brushing their teeth but I do think they should be brushed and free of cavity causing food and germs. Just my opinion though.