NO CHILD LIKES TO BE TOLD WHAT TO DO, especially in public. If your child has a habit of eating with their mouth open in restaurants, then drawing attention to it – especially if it means doing so in front of grandparents and other children – will simply make them feel bad.
Instead, try a subtle gesture, such as pinching your own lips together with your thumb and forefinger. Try to do this while they’re looking your way, and no-one else is.
Children usually want to behave well, and appreciate being helped to do so. But spare their feelings, and don’t make everyone else around the table aware of it.