Do some kids have weaker teeth than others or is it my fault they have cavities?

Hygiene & nutrition: Genetics can play a role in getting cavities, but that's not something that is controllable. What is, however, is whether kids are properly brushing & flossing, seeing a dentist on a regular basis & also what they are eating. A diet consisting largely of sugar or starch can hugely influence whether he/she gets a good checkup or is getting cavities. Supervised brushing is recommended for kids < 10.

Cavities are complex: Dental caries is the result of acids demineralizing the tooth, though some people seem to be more vulnerable. Acids come from bacteria metabolizing the foods you eat ( stay away from sodas, sugars and carbohydrates) or from acidic foods like diet sodas. A little Fluoride ( in water and toothpaste) can help remineralize the early stages of demineralization and make the teeth more acid resistant.