baby teeth and cavities

Hello!
My 1.5year old has mild to moderate tooth decay (she nurses inthe night, and well, we were too slack with brushing) We brush regularly now (2times a day) with New Co's kids toothpaste (the only kind i found free of crud ingredients, flouride free and ade in canada!) And I try to rinse her mouth after night nursing, but that usually doesn't work(but i good luck with the syringe like thing they give you after having your wisdom teeth pulled...we lost it though)
My question is: Is there anything I can do to help STRENGTHEN her teeth?
I've watched her cavities spread from one tooth to four, two teeth in only two weeks were affected! It's her front teeth and one molar, and I'm really concerned about it - she keeps her molars til 11 or 12 years old!
Why are her teeth so susceptible to decay? She doesn't get hardly any sugar, or juice. No sippy cup. So why?
Thanks Tweet