Your daughter may have had a refractive error
earlier but it may have been lower, hence the complaint was not obvious then. It is possible for a child to develop a myopic number in this range at this age. Children can develop a refractive error even if the parents have nothing significant. What is important is that you encourage her to wear her glasses at all times. The vision may be subnormal and should improve but only if she wears her glasses religiously. There is not much you can do to reduce or stabilise her number. Teach her to practise good reading habits like reading with adequate light but her number will increase as her age advances in spite of this till 18 or 19 years. Later on, you may try contact lenses and after the age of 19 when her number stabilises, LASIK or laser
correction. None of this is indicated in a child though.