There is a reason signs like this are posted in zoos and parks. Certain behavior should not be encouraged. Regardless of how cute or fun the behavior seems at that point in time. I wish people would not feed my monsters. Or in other words I do not want them to encourage certain behaviors that turns my angels into monsters.
For example, if one of my children smack you, headbutt you, pull your hair, scratch you, bite you, and the list could go on... don't laugh or do anything to encourage the behavior. I can not stop these undesirable habits if I am the only one telling the kids no. It is not cute. It is not funny. Maybe a head but from a one year old might seem funny to an adult. It probably doesn't hurt much. However, imagine how that same head butt feels to a two year old.
Every time you allow a child to act this way you are saying, "it is ok to do this!" They look to us for guidance on what is acceptable behavior in society. They do not understand, "It is ok to be rough with daddy, grandpa, uncle, or whoever, but not sister!"
So what do you do? That is the easy and hard part. Talk to them. Tell them, "Ouch, that is called (whatever they have done), and that hurts!" If they continue to do it, put them down. I tell my kids I will not play with you if you are going to do that (whatever it is). I am not saying they are perfect, but they know mommy means business!
Give them a little time out. I have been using time outs for my kids starting at 9 months.
Evie gets about a minute in time out, and Christopher gets about 30 seconds. It is usually much less. The point is to make them take a second and understand this behavior will not be accepted. It is not fun for you or the kids. Nobody likes to be the meanie, but allowing this behavior turns your angel into the meanie! You know who he is, the kid that nobody plays with because he isn't nice!
The hard part is you have to be consistent and you can't laugh at them! Ah, there's the rub! Don't even smile at them. Ignore the cuteness, and bite your lip or turn your head. They will devour you if they sense weakness! I am not kidding. My kids already know us well enough to know if we think they are funny. If they get that impression all is lost! Who doesn't want to make mommy or daddy smile!