Thanks for posting your query at HealthCareMagic. I went through your the details provided by you. I suggest you not to worry much. As a child approaches adolescence, development of sexual attraction towards any sex is not abnormal. The explanation is simple genetic functions of our body alongside the exposure to various external stimuli from around us. The young mind often does not know how to deal with them. As time progresses the body and sexual mind understands how the system works and adapt accordingly. As you mention, she counts the mentioned thoughts as 'dirty'. This clarifies that she has already been exposed to the distinction between healthy and unhealthy things with respect to sexuality. It might simply be that when she is becoming more conscious about her own body, she draws her references from the people around her and she becomes bedazzled by the taboo around the so called sexual organs like breasts which she is yet to develop. You do not have to worry. Reassure her and tell her just to forget about these unnecessary thoughts. Proper reassurance is all that is required rather than further burdening her with what and what not to do. It is only when the problem is beyond control that she would need to go for psychotherapy.

I hope that would help you intervene into the situation. Feel free to revert back in case of further queries.