Pat,
I feel your pain, both on the needing to care for the children, both from the daughter standpoint as well as the grandkids standpoint.
Speaking as a former user of heroin, you ultimately have to try to step into your daughter's shoes, just for a moment, no matter how upset you may be with her. From experience, I can tell you what heroine does: it makes you feel like everything is peaceful and there are no worries at all. Everything seems to be at peace in your life, and you want to just sit there and relax, soak it all up.
It's a sense of peaceful euphoria.
The downside, of course, is waking up the next morning after the drug starts to leave your system. It HURTS. Picture the worst hangover known to mankind and multiply that by 1000. If you ever had a migraine, imagine that all throughout your body. You want to curl into a ball and die, anything to make that horrible pain stop.
It's even worse when you have loved ones on top of that, and shame, paranoia, and pride all take hold. You know you're hurting them, and your body is telling you to stop hurting it and get some more. Heroin is a lot easier to get than you would think. My habit was free because my friends were junkies, and when they nodded, they wanted company. I could pretty much get anything if I was in the right place at the right time. I swore that the day I needed to actually PAY for the drugs was the day that I was officially a drug fiend. Little did I realise just how fast you get addicted to heroin. Your body goes through DT's from just one dose. I'd been a regular user of prescription drugs before that (they were prescribed for my stomach problems, and just general anxiety attacks), but no one told me they were addictive, and the bottle said take as needed. Next thing I knew I was detoxing when I tried to stop taking them. Heroin detox is a lot like that, but worse.
I took heroin because I was miserable at the time. I'd heard that you coul overdose on the first try, and it made no difference to me because the way I looked at it, it was a way out either way.
I made it through my addictions with help from my family. If they hadn't been there for me, I most likely would have killed myself through overdose instead of just going through the detox pain. They gave me something to live for. I understand that you're probably furious with your daughter for doing what she's done, and it's understandable. I was mad at myself for the longest time. Especially after seeing friends die from overdoses.
What I'm attempting to say is, first and foremost, try to understand your daughter's pain. You don't have to stop being mad at her, just try to be there when she comes back around. It won't be easy. It wasn't easy for my mother, and I didn't even have kids. I'm happy to say that I've been drug free for about two years now. The past has been left in the past, and I've gotten on with my life and a successful career. It wasn't easy. You have to want to succeed at it. You have my support and I'll be happy to give you my email address if you want to vent to me. Just keep a smile for your daughter and show you support her, and she'll be alright in the end. This timeframe is going to be the hardest of all, for both of you.
As to the children, this will be very rough on them as well. Try not to vent to them about their mother. If your daughter really doesn't want the children, then by all means ask her to give you custody. If she wants them, however, you taking them away might only make her problems worse, sad to say. From my own standpoint, I've been as careful as possible regarding birth control, etc., because I've always felt that if a child comes from me, it should be intentional and wanted.
Ending, just from the standpoint of a former druggie, don't give up hope. With love and caring, you, your daughter, and the children can get through this. It will be a rough road. The end of the journey is worth it, however. My mother and I are closer than we ever were before. Once I got straight, I saw things from her perspective, and I realised how selfish I had been. We gained a better understanding of each other.
For the immediate problem that you face, the two week vacation might not be a bad idea. If possible, try to re-open the lines of communication with your daughter, and let her know that you're there for her, but you won't be a doormat. Tell her how you feel, and ask her for her point of view. I think you'll find that she is most likely in the same frightening place that I was in not all that long ago.
Is there any way you could possibly hire a babysitter instead of a daycare? Someone you know in your neighborhood who could use the extra cash? Someone you trust, who you know likes children?
I hope everything works out for you, and I'm keeping my fingers crossed for you and your daughter, and the children.