We DID send our (then) almost 4 year old to preschool because of his aggression to our little one. Although it was a good school (Reggio Emelia,) it broke my heart to leave him there. After 2.5 months we figured it all out and were able to stop sending him. Well, the problems had gone on for about a year before we figured it out, but he was only in preschool for about 2.5 months.
Yes, a lot of it was about attention. We got help from a local parent education center AND in home therapy. We thought the problem stemmed from our abrupt adoption of a newborn. We were sitting on a bench, got a phone call, and less than 4.5 hours later I was nursing our new daughter. Although we knew we wanted to adopt, we didn't tell our 31 month old because we weren't sure if it would happen or not. So, from his perspective it was a normal day then all of a sudden Mom and Dad leave, then Dad comes home and says Mom is taking care of a baby.
Anyway, when he was just shy of three and we had just finalized the adoption, he became HORRIBLE. I would wake up in the morning and think how much I had once loved him and how he was now the enemy.
Fast forward about 9 or 10 months...
What initially worked best was to ignore him. If he started hitting or shaking the baby, I would CALMLY pick her up and CALMLY say, "When you are ready to play gently, let me know," and I would CALMLY walk away. By not over-reacting, it defused him. However, it didn't stop it altogether. What I started doing was, whenever he would be aggressive, I would take him into the bathroom...he was usually kicking and screaming and I tried to be as CALM as I could. Then I would block the bathroom door and tell him he could leave as soon as he could play gentle with the baby. He would head butt me, hit me, scream, kick, try and get past me, etc. It was awful and I hated blocking his way, but I was desperate. I totally did not react to all the physical stuff he was doing to me. I would just randomly say, in a CALM voice, "When you are ready to play gentle with the baby, you can leave." Eventually he would say he was and I'd move out of his way and he would be gentle. This only took a few days to be effective. I HATED doing it, but nothing else had worked. I have since talked to another mom and she said she did the same thing and it worked.We have not had any problems with aggression, beyond fairly normal stuff, since.
When he was a bit older, we had a lot of problems with him whining. It was awful and enough to drive everyone up the wall. After talking with the parenting education center, I sat him down and told him. "From now on, when you whine, my ears will get turned off. As long as you are whining, I will not be able to hear you. When you talk in your normal voice, I will be able to hear you." As soon as he started whining, I made a motion near my ear as if I was turning off a switch and looked away from him. The whining, begging, pleading, etc. really escalated. It was so hard to ignore him. "Please Mommy, I need you to listen to me." In an awful whiny voice. As soon as he said anything that was reasonably normal I responded. He still escalated the whining for a couple days. By about the third day he pretty much stopped it. He randomly gets whiny from time to time, as many kids do, but all I have to do is tell him I am going to turn off my ears and he talks in a normal voice.
I wish being gentle was enough to take care of the problems, but they weren't. Although I hardly had to do anything draconian, I did have to do some persistent non-attention giving to break the cycle. The woman at the parent education center said children are starved for attention. If, in their minds the only way they can get attention is through negative behaviors, that is what they will do. I am very attentive to my kids (and I only have two,) but for him I wasn't attentive enough.
I hope this helps.