This ^^^. I'm a parent of 3. Kids get black eyes for a million things. My 3 yr. old daughter fell off her chair today and hurt her wrist while eating her lunch. How did she fall? Who knows, she's three and has a talent for falling. So the black eye thing means nothing. I mean it REALLY means nothing.
In your scenerio, you said it was a Starbucks. That alone would lead me to believe it wasn't an abduction. Someone abducting a child, doesn't stop off at a Starbucks, and leisurely knock down a cup of coffee, with lots of witnesses around, but I suppose it is POSSIBLE. If you would have said a convenience store, at an odd hour at night, that would be more suspicious to me