R.R.
I know it sounds rash, but take away ALL privileges. If they have cell phones take them away. If they are used to hanging out wtih friends put a stop to it. If they have a tv in their room take it out. If they have toys take them away and replace them with books. Again, I know it sounds rash, but it worked for my 7 year old ADHD child that was misbehaving in school and being extrememly disrespectful to me and the teacher. I boxed up EVERYTHING he has except two stuffed animals and his books. You wouldn't believe the change in him overnight. I let him earn his toys back slowly. One day of good behavior and I let him play his gameboy for 20 minutes before bed. One week of good behavior and I let him have one toy back to keep. If he went back to bad behavior for just one day I would warn him that the next bad I would take away the toy he already earned back. On the other hand if he has a whole month of good behavior (I give him one day a week as leeway) then he can have back several toys. He has lost toys he had already earned back and he hates that. This also means no more buying toys when we go to the store. I am not going to replace his toys that were taken away with new toys. Plus by the time he earns a toy back he has has missed it and ends up actually playing with it instead of just leaving it laying around. TYR IT!!! IT HAS WORKED for me so far and I highly recommend it. Spanking doesn't work. Once a child realizes he can live through a spanking he will do what he wants when he wants.
I wish you luck, I do not envy you having to care for 5 children and having two of them treat you so poorly.
R.