A stranger slugged a grandmother as she took her grandchild for a stroll this week — punching her in broad daylight in an unprovoked Queens attack, cops said. The 65-year-old victim was pushing the child in a stroller around 4:40 p.m. Wednesday on Kissena Boulevard near Franklin Avenue in Flushing when the menace struck her in the face without saying a word, authorities said. The blow left the victim with facial pain causing redness and…