Have you tried having a conference with both parents about their children's behavior? It sounds like the dcps need some education and you all need to become a united front with these 2. Consistency and unity. Decide on the type of guidance and discipline you all want to use and then stick with it. Maybe give it a month before discussing progress, then make the decision. Of course, this is all dependent on if you want to give it a try with the dcks or if you're just completely done with them. And there certainly is no shame in that at all.
I have to be honest, I have kept some dcks way beyond the time I should have, but then let others go before I worked very much on the situation. It depended on the dcfs, if they would work with me, and it also helped if they weren't FT(I got a bit of a break with them).
I let a dcf of 3 boys go, they came Saturdays and the whole family was a wreck; but the last straw was when the 5 yo called me a f'n bitch. I called for pick-up immediately. And was done. But I kept sibs who were difficult due to being high maintenance; they were physical and needy and I over-kept them because, ironically there was a bond there.
Michael's right; it's entirely your decision and what you are ready to put up with, try to work on, or done with.