any suggestions

on what to do when your child is mean to your other child? I don't know what is worse. 1. The older child picking on the younger child. Or 2. The younger child picking on the older child. It's pretty crazy when I have to deal with this..and my children are under the age of 3! And, when it's the YOUNGER child picking on the older one!
I just don't know what to do with Preston. Constantly, all day long, all I hear is, "Preston, don't hit me! No Preston, No!" "Don't hit me Preston." "OOOWWW, Preston!" I look over and Preston is hitting Parker on the head with a car, a ball, a spoon, a paint can, ect. ALL DAY LONG! What do I do? I have tried time out with Preston, but he JUST turned one. I tell him to tell Parker he is sorry, and to give Parker a kiss, and he gives Parker a kiss. Errgg...it is so stinkin frustrating!
I was trying to cook dinner earlier, and Preston was hitting Parker, Parker was crying and getting frustrated, so I asked Parker if he wanted to go into his room by himself and play, but he would have to shut the door. He thought that was a great idea, and went running into his room. (Which is a huge improvement, he used to NEVER entertain himself.) No sooner do I shut the door, Preston comes running down the hall crying because he wants in the room. He is screaming, loudly, but I keep my promise to Parker and let him play by himself in his room. Preston continues to scream and I go about making dinner. (Hey, at least they aren't fighting!) Then, , Parker opens the door to his room, and says, "Come on Preston. Come play with me." It, to me, was one of the sweetest things I have ever heard. Maybe it's because I am pregnant, maybe it's because if I was in the same situation, I would have made sure the door was locked, but it was just so sweet and forgiving on Parker's part. I just love both my boys so much! :)