Both our kids ended up with croup a couple of months ago at separate times. We were much better prepared with the 2nd time. Our son was pretty bad by the time he was able to get into the doctor. She have him a shot of a steriod that started with a d. That was all the steroid he got. It was the end of the day and she sent us to ER for him to be watched. A couple hours later and we were released. They told us to us a humidifier in the house/his room and if the cough gets bad expose him to cold air (outside or open the freezer). It was the cold air exposure from the doc office to ER along with the shot that got it under control so quickly. With our daughter, she wasn't nearly as bad but I picked her up from daycare and heard the cough. Urgent care didn't have any treatment for croup so back to the ER we went, she got an oral steroid and then we did the same for her.
Since your daughter gets it more frequently and it's a mild case, try the humidifier, we also boiled water on the stove, and regularly expose her to the cold air, especially at night since croup gets worse at night. If you do this, hopefully she won't have to go to the doc and hopefully she can avoid the steroids. I would want to keep her immune system up too.