Good evening, I would like to be of assistance to you today.
Seeing that you are dealing with several different children, with different personalities and ages, you may have to approach the form of discipline that is used for each one accordingly. As you yourself have noticed, 123 Magic only works with the oldest sometimes. The foundation of providing order in these youngster's lives is first with sitting them down and having a discussion with them about the rules of the house. Use this discussion as a time to teach and interact with them, ask them questions like, "what kinds of things can we do together to have fun and stay out of trouble?" They may provide outlandish recommendations, but at least you will begin a dialogue with them that shows them that you want them to be good, and they may even be able to give you some good insight to things that would keep them entertained and out of trouble. They are definitely old enough to get involved in little league sports or clubs; this would be a good time over the summer to get them into some organized activities to help teach then focus, cooperation, and good behavior. When one of the children begins to scream or hit, calmly stop the behavior and explain to the child how hitting and screaming is no way to get what he or she wants. Give them recommendations of alternative ways to get what they want, through asking, good behavior, cooperation, etc.
While it is necessary to set punishments for poor behavior, it only works with giving rewards and praise for good behavior. When you find the children working well and being good together, stop to take notice: "I am so proud of you all, you have been good all day today, I think this calls for pizza!" Do these things just because they are being good, not as a bribe to get them to behave. When they do misbehave, set a clear punishment: no tv, toys, outside, time out may work for them at this age; and be clear in explaining to the children what kind of behavior you expect for them in order to get out of the punishment. Remain firm in the punishments you set, but also be flexible in regards to what the offense was and how well they improve their behavior. When you are correcting behavior, work to refrain from telling them no all the time, but instead let them know that what they are doing is not the appropriate way to behave, and give them options of how to improve their behavior. If one of the children is throwing things, explain that throwing things is not the right way to express anger because it breaks things and people might get hurt. Give them a choice of other options that they can do to relieve frustration. If they have done bad enough to get put in a time-out, let them know that after their behavior is improved they can have an option of doing one of the alternatives to releasing their frustration (painting, singing, dancing, running, etc.).
There may also be times with slight misbehaving that the best option is just to ignore the bad behavior. Children often do things (good or bad) in order to get attention. Don't allow the negative attention seeker to win. I would also recommend that you are sure to take the children regularly to the pediatrician so that he or she may be able to diagnose any possible health related issues that may be a cause of either child's bad behavior. Here is an article that provides further detailed recommendations of how to discipline your children and help them to develop self-control:http://www.education.com/reference/article/Ref_About_Discipline/?page=3
If you need any clarification on my answer, or a more in depth response, please do not hesitate to message for earthsister.