My 21 month old daughter was accidentally hit in the head with a 10lb dumbbell. She has a knot on her forehead the size of a quarter is it OK to let her sleep or should I take her to the ER?

Are you kidding me?

First, how is it that she got accidentally hit with a dumbbell?

Second... why are you sitting around asking people on the internet if she should be looked at? Of course she should be.