the link between calcium as a child and the strength of adult teeth is not proven. if your teeth are cracking it's probably the result of failing fillings. the most important nutritional advice i can give you is to reduce the ph or acid content in your mouth and avoid soda which leaches the calcium from your body. xylitol gum anti-caries rinses and fluoride supplements will all "harden" your teeth and make them less susceptible to cavities.
i didn't get enough calcium when i was younger. is there anything i can do to make my teeth less likely to crack?