Your granddaughter's play concerns me. Children this age often play out problems in their life because they have limited language skills. I'm worried that someone may be mistreating your granddaughter, or that she has observed some violence. I can't be certain either is the case, but I recommend you start looking into this situation.
Check her body for marks. Ask her such non-leading questions as: why is the baby being hit? What did she do? Do you know any grown-ups who hit their children? Does anyone slap you in the face? Approach her at different times with these questions, and see how she responds. I wish I could put your mind at ease, but there's a possibility that her violent play is a reaction to a real-life event.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.