hm, well this is an interesting case. While I feel for the grandmother she did smack her. I dont think you can legally smack a smaller child in the face either. Corporal punishment usually involves spanking on the butt, not smacking in the face.
I understand why the cops arrested her. From the article...
"If an officer on scene finds probable cause to arrest a person, because they've committed domestic battery, then our policy is a mandatory arrest of that person," said Lt. Loux, "The discretion does not come because the victim does not want to prosecute. It doesn't come because the victim has remorse for calling police
How many woman who are battered by their husbands beg and cry not to have their husbands arrested? lots of them. Its up to the police to just do it. Where do they draw the line? Arrest the husband but not granny? When you have police officers picking and choosing who they are going to arrest based on whether or not the like the person, and lets be honest, everyone likes grandmothers, there is going to be a BIG problem.
More than likely the charges will be dropped against the grandmother and hopefully the family can work on getting back to being respectful of one another.