I'm going to grab a kid in the grocery store by the shoulder to stop them from being annoying
What are you going to do with the kid that you plan to grab by the shoulder?
I'm just going to explain to them that you're not allowed to carry on in the store.
You should not get involved with it. It's not right for you to lecture or parent someone else's child or it would look like you are assaulting's a child which might cause a public disturbance.
How else can I stop her from talking? Everyone is staring at me.
You should never touch children, especially those that aren't yours. You can speak with their parents, but not the child themselves as you're way too old for that and it will be a serious breach of human etiquette.