Well depending on the childs sight you can do 1. Nothing or 2. Something right away.
Honestly the child is old enough to care for them, but adults loose glasses also. Unless its a consistant thing then you chalk it up to having kids in glasses. If they loose them again then they need to find a way to pay for them themselves. But one pair lost every once in a while is typical.
My 14 year old lost his glasses. After searching for 2 days I bought him new ones b/c his vision required that he have glasses to function properly in school. SO I sucked it up and bought new ones. Then found his the next day in the fridge..swear to God.
If my daughter lost hers I would have to buy new ones immediately. It depends on the kid and the script.

Jenna and Michael, lucky to have Adam, Mady, Aidin and Bryn.