To make a long story short, the other night my MIL was baby sitting my one and a half year old niece and came over to our house with her. During her visit, my niece slapped my three month one in the face and no one was quick enough to stop her. I panicked and didn't know what was appropriate to say or do. I was furious but didn't lay a hand on my niece. My question for everyone is: What is your best advice on how to deal with a young child hitting another child? And how do you deal with your initial anger over your baby being hit so you can respond in a calm fashion?

I know how I'll be reprimanding my own daughter for this in the future if she hits another child, but am unsure how to address it if the child doing the hitting is not my own.