The simplest answer to this is sugar. Foods that contain a lot of sugar and not good for kids teeth. Sugar helps the bacteria and plaque in the mouth to break down teeth and cause things like tooth decay. Things like orange juice are also bad for teeth as the acid breaks down teeth over time.
Foods and drinks that contain caffeine can cause teeth to go yellow, so things like chocolate and Cola (which contain both sugar and caffeine) are not so good for teeth. If your child has only small amounts of these things and cleans their teeth every day, then they will reduce the risk of decay and disease.