I'm not a parent, however, the break-up could have set this behavior off. What you can try, is something we learnt in 2nd year university for abnormal psychology. It can work for children, and possibly adults, the Token Economy. Basically, whenever the child does good behavior, you give them a token(s). When they have a certain set amount, they can "cash them in" in order to have more freedom or privilages. This requires you to set up what is good and bad behavior, how many tokens are given and how many can be cashed in for different activities.

In university, and when I go into medicine or research, I do little coursework with children. So, I would assume that you should not make it too difficult to get the tokens or set an outrageously high amount of tokens for an activity or else they won't participate.

I'm not sure what they kick and scream about, however, from what you've posted, it seems like they control you: they make a fuss, you make a fuss over them, they probably get whatever it is. If you don't give in and stay true the Token Economy, then you can gain control: they make a fuss, you don't, they don't get what they want but they make no fuss, you control the tokens and activities.

Also, typically the child whose younger follows the older one or they work in pairs. So, you could try to give tokens if both do good behavior, however, conflicts can come from that. But, the point is, not to favour one over the other.

The problem with the discipline is to them, you are being hypocritical. You tell them don't hit/yell yet you do it to them. Kids tend to learn via observational learning, so if they see/have you hit and yell, then to them, it must be a norm to get what you want. Also, when they do it, it works. So, why stop doing something that works? You have to come in, don't encourage their behavior but rather put the foot down and show you are the boss, not them.

There are other ways aside from the Token Economy, however, I remember this one from my lectures and textbook as being pretty effective. For the tokens, you can use whatever, but, not real money. First, it'll drain your wallet, second, you have no control over what they do, as you become more or less an "ATM mommy".

I cant be of much more use. Next year (3rd year university), I will take abnormal psychology for children, among other courses, but as of yet, all the biology and psychology courses have focused on adults.