Rear bumper pierced by low-speed rear-ending

Argh!!!!!!: our 6-month-old Prius suffered its first injury today. We were tapped on the rear end by a Volvo wagon. When I jumped out to look for damage, I didn't see anything, so let the other driver go on her way. But then I pulled over later for a closer look, I discovered a tiny circular hole, probably made by screw holding on her license plate. The hole is maybe 1/4" in diameter. There are no other dents that I can detect.

My questions:
Can this hole be patched or otherwise filled in?
Or does the whole bumper need to be replaced?
If this is the case, should I file a claim with our insurance company?

And, until I can get the hole repaired, can water/moisture/snow/ice getting inside damage the bumper? (We're in Chicago--plenty more bad winter weather, ice and salt lie ahead.)

While I know this is a relatively minor thing, it's still the first damage suffered by our Eva, and I feel awful about it. Deep thanks in advance to anyone who can help me out with answers.