An occasional glass of soda or of cake or ice cream is not going to do her harm.
What does she eat regularly? Some things that appear healthy have more sugar and calories than you realize. Many "juices" are high in sugar and calories. Do you have processed foods in the house? Those are bad and can cause extra weight gain.
Focus on having healthy foods - those with minimal processing. Fresh fruits and veggies are the best. Focus on being healthy rather than calories or you cause your daughter to have an eating disorder. Regular exercise will also help. Get some bikes and go out biking together, go swimming - kids love swimming, find a sport or activity that she likes and join in with her and get her signed up for a fun recreational sport or activity.
Depending on her age she could outgrown this - some kids are chubby when they are young and as they become more active and go through a growth spurt they thin out - not the case with everyone. If you are really concerned I would check with her doctor for advice and the doctor's thoughts.