I fixed them by putting two darts in the back of her waistband. This is beyond easy. Why mess with putting safety pins in when you can just fix it for good. The easiest way is to turn the pants inside out and have them try them on. Then pinch in two places on the backside of the waist. I usually do it about an inch and half from the middle seam on either side.
Now just sew it closed.
You will need to make sure you angle it. Here's a picture showing the angle with the contrasting thread.
Now we are ready for some soccer!