As a child, my parents took me for an eye test after my Mum was helping me with some homework. She picked up something to show me and I laughed and said "hold it further back - it's too close, I can't read it there!".
I ended up having glasses particularly for close up work as I was long sighted.
Wasn't a surprise - my Dad is particularly long sighted.
Now I'm 44 and wear glasses, but I'm now short sighted, so have to wear them for driving and don't need them for close up work! Again, not a surprise - my Mum is particularly short sighted!
Long story short, eyesight can change a lot over the course of your life.
I went from being +whatever when I was a child, then through to -1.75 as a 30-odd year old and now I'm at -0.50 and heading back to having good vision, before I probably end up back with reading glasses!
If none of your family wear glasses (other than age related reading glasses), I can see that your child suddenly having to wear them can be a bit of a shock and you may be more upset that you didn't realise, rather than the fact that she has to wear them. Maybe your DD is upset because she thinks wearing glasses will make her stand out. Is there anyone else in her class that wears glasses? Do they get picked on maybe?
I agree to sell it to her as a positive thing and point out that she'll now be able to see everything much clearer.