Thank you for asking your query at DoctorSpring.
The aim of corrective glasses is not all about seeing , it is also about seeing clearly and effortlessly reading. Now a child might be able 'manage' without a glass, but she will be limited and troubled her reduced vision, which she might not even realize. This is important in her proper neuro development. So if the child is having a power of -3.00 , it is mandatory to wear glasses constantly & remove it only during the time of sleep.
Hope this helps.
Please do feel free to ask followup questions.