P.P.
My 21 month old daughter gets a good laugh out of hitting mommy and daddy in the face. When she starts hitting I gently grab her hands and say "we don't hit!" or "no hitting!" I then explain to her that hitting hurts mommy and it is not nice. I then tell her "gentle, gentle" while gently stroking my face with her hand. The next time she starts hitting I say "no hitting, gentle, remember?" She remembers and starts stroking my face again. I feel you just have to train them it hurts and they should be gentle. I tried punishing her by hitting her at first, but she just hit more because she thought I was condoning her hitting (by hitting her). I find that children can be rational beings and sometimes respond well to talks or explanation rather than punishment.