If he is only around older siblings he is likely being rough housed with by them and the parents don`t really realize what is going on because they know the kids are just teasing and playing. The problem is that the 18 month old doesn`t. As a result he things this is how one plays. The problem with saying use gentle hands etc is that that is just words with no meaning to the child. But he is obviously being turned into a bully either by siblings or because he feels he has no other way to compete. There is also a good chance he is spoiled at home and the older siblings regret it and take it out on him - especially when the parents aren`t watching so that is all the child knows. But a different daycare setting with lots of older kids or whatever might just work for him where he can meet his match so to speak. The great thing about an abundance of daycare providers is that there is a situation for everyone. Just too bad for those of us looking for care that there isn`t a child for every space just when we need it. Good luck on filling the space soon.