As a nearly lifelong glasses wearer, I'd strongly caution against buying glasses online. Fit is critical, especially for a tiny person who's growing and developing so fast. I would, however, buy at Costco in a heartbeat.
In fact here's a cool Costco story. It's paraphrased, but recent and true. Two Costco employees brought their munchkin in to have her eyes checked before school started. The in-house optometrist (not a Costco employee) did the exam and saw something he didn't like. He did more tests and then more tests. He sent the child out of the office and brought in both parents. He advised them that what at first appeared to be a shadow was actually a tumor snaking from the child's brain and starting to wrap around her optic nerve. He directed them to a specific hospital's emergency room, wrote down all his findings, gave it to them and insisted they go straight to the hospital. Fortunately, this story has two happy endings. The little girl had surgery to remove the tumor (she had no symptoms) and has completely recovered. The diligent optician? Costco gave him their President's Award, created to honor outstanding employees. Did I mention he's not an actual Costco employee? Yay for him, yay for Costco for recognizing him and yay for the family and their healthy little girl.
This event, plus my own experiences there are why Costco will always get my vote. Oh, did I mention that they have great prices? Yeah, that, too.