PG, I know things are busy and right now you are SOOOOOO sure that difficult child won't ever live with you, you won't put up with her koi ever again, yada yada yada. I am BEGGING you to either work with a therapist or go to 12 step meetings on a committed and regular basis.
difficult child did not develop a drug problem all on her own. Family dynamics had a HUGE role in her choices. This does NOT mean her drug use was your fault, but it does mean that the entire family, ESPECIALLY YOU, developed very unhealthy habits of codependency. Don't give me that, "But I know I won't do that again, my mind and feelings are so different now" song. You have to understand that the change in your feelings, etc... are all very much to be expected with codependency.
If you go back and read the entire history of your time on the board, whatever is archived, you will see a real pattern in behavior - yours, husband's an difficult child's. I think that right now your feelings really have changed because you saw how difficult child harmed her child with her koi. It means that right now is the time that some hard work on your part could play a HUGE role in little C not learning the patterns that are so ingrained in your family and parenting style. NOT that you are a bad mom, but that addiction permeates every facet of your life and has a huge impact on all of it. If you truly want little C to have a real chance in life to avoid his mom's mistakes, you will go to meetings and/or a therapist on a committed and long term basis. Not for a few weeks or months - go for a couple of years.
I know how hard it is with a small child. I went to alanon starting just before I got pregnant because my mother was facing her father's addiction and how it impacted our family. Now my Gpa NEVER lived near us and never ever drank around us. I never saw him drunk. But I started going to meetings because it was important to my mom and because at first she did not want to go alone. Then I found out I was pregnant and I did NOT want my child to learn my Adult Grandchild behaviors Plus I admitted that my bro had a real problem and I wasn't 'just' an adult grandchild - I was the sister of an alcoholic. To my husband's credit, he not only didn't mind that I went to meeting, he went to some also and admitted to a similar family history. It was NOT easy to find the time or energy to go to meetings, esp as we liked separate meetings, but it helped SOOOO much. I think you will find the same thing a few months after you start going to meetings.
I very much am a better person and parent because I got help. I think you are not aware of the things your difficult child may do if she thinks you will not continue to be her 'cushion'. She knows that getting pregnant got you to give her a whole lot of support and that one grandchild was enough to get you to turn a blind eye to her drug use for quite a while. What will you do for the next child she has? What is to stop her from getting pregnant again as soon as she is out and learns that you won't let her come back home? How will you keep a firm boundary?
You are feeling very strong in your desire to protect little C and give him a safe and happy addiction free childhood. You will NEED support to keep that determination, esp when his mom starts tugging at your emotions.
Please, for little C's sake if not your own, work with a therapist or a family group like alanon to give little C the absolute best chance in life that he can have. He needs you to do this.