I don't know if there are any female cardiology attendings on this board.
Don't worry...you have plenty of time to discuss this issue with cardiologists (female) once you start medical school. Many people have these concerns. There are several (though not a lot) female cardiology attendings at the place I did residency. Most of them do have kids. One is even an interventional cardiologist, so it can be done.
Currently only about 18% of cardiology fellows are female, and probably a lower percentage of practicing cardiologists (I'm assuming the latter because in the past the % of female fellows must have been even lower). I would expect the number to go up a bit, since it seems like there are a lot of women medical students interested in cardiology, and there are more and more women going to med school all the time.
If you choose to do cardiology, you will probably encounter occasional negative feedback from some cardiology attendings you may work with (or who might interview you for cardiology fellowship, etc.) but these people are by far not in the majority. If you decide you want cardiology, you'll ignore these types, most likely.
Cardiology does tend to work longer hours than some other specialties, but from what I have seen the hours are definitely better than some surgical fields (like general surgery, or any type of trauma surgery). Interventional cards takes a bit longer to train for, and tends to get more emergency/on call cases, so they have the worst/longest hours among the cardiologists.