That right there may be the source or at least adding to the problem she is having. Does HW know she is using polyester strings. To much for a 10-11 yo to handle and not necessary especially now as she is going through a grip change. Consider this setup to soften things up a bit. Stay with the Black code (technifibre brand), 18G OK or can go with 17G, put them only on the mains. For the cross string consider using Technifibre X-One Phase (multifilament) 16G. String the mains 4-5lbs looser then the cross strings ex 50Lbs mains w/ co-poly, 55lbs cross w/ multifilament. If your daughter is playing at least 10-12 /wk then you will probably restring them every 2-3 weeks which is what you would need to do anyway if you are using only the polyester strings. This is done whether they break or not.