Trying to hide it will just make them suspicious and more likely to fire you. Tell them what happened, completely. It sounds like it isn't your fault.

How about calling her parents (Mom at least) so she/they aren't shocked when they get home/ pick her up....

Call one of her parents and tell them what happened, but dont sound scared while your talking to them. tell them that she is sleepnig now but that she has a a bruise. they will trust you more if they know that when something happens you will contact them.

As parents, they probably know that this kind of thing happens to kids. It doesn't sound like it was your fault, and they've probably had the same kind of accidents happen while they were with her. No big deal. Let them know what happened when they get home, and don't worry too much about it.

Kiddos are gonna get bumps and bruises. If I were in your situation, I would call one of the parents and explain what happened. If you dont feel comfortable calling... Let them know as soon as they get home or pick her up. My nanny just texts me if something happens or she tells us when we get home. Be honest, they will appreciate it.

I would notify the parents of what happened, and any true parent would know that accidents happen. I have a 2 yr old who falls and bumps into stuff all the time. Just yesterday at the playland inside Mcdonalds, she was running with some other kids, wasnt paying attention and BAM, smacked right into a chair. It happens... But if it's a head injury, thats nothing to blow off.