I got a bit of this when my daughter got them. I was a bit WTF as well!
One friend I said "oh, xx is getting her glasses next week!" and she said "ooooh, never mind" as if it were something distressing.
I said to my brother via a message that she needed glasses (part of a general conversation) and he said "aww poor xx"
Even husband said, "oh, does she?!" in a disappointed voice when I rang him from the hospital apt where her vision thing was assessed.
Anyway fast forward about 3 years later and I think everyone realises it was a total non-event. The glasses they have these days are quite sweet (worse for me when I was a kid growing up, very plasticy looking and thick!) but its all characters and cute roundish frames and stuff, I think she almost looks better with them. In fact she does look better now, because she had some squinting/lazy eye issues before and now she doesn't, due to the glasses and some patching we did.
Her prescription in her bad eye is worse than your childs, but the difference between the good eye and bad eye is about the same, which is what causes the squint (the "difference"). Actually it can also be a muscle thing, but the discrepancy is what causes them to stop using the eye eventually.
Anyway, total non event, and if she wants to do without them when she is older, because of sport, dancing or vanity, that's fine, she can get contact lenses on the NHS, and they don't have to wait till they are 16 or some magical number, age about 10 is fine apparently.
I had specs from when I was about 14 months old, and they were the least of my worries, in that I needed 2 operations. I mainly wear contacts now, and sometimes glasses, and I'm finding now I'm in my late 30's everyone around me is getting glasses anyway and when I'm wearing my contacts I'm one of the only people without them!