I have to make some assumptions here because this is a legal case and I don't have a lot of information to go on from your question. Since your daughter lives with you, I am assuming that the grandchildren do also? I am also assuming that whatever you have found could be considered dangerous to their welfare, such as drugs, weapons, etc. so please correct me if I am wrong.
Family law is complicated and varies from state to state. The fact that they are living with you is a complicating factor. So in getting custody of them, you want them to live with you, but not your daughter, right? So you can ask your daughter to leave your home whenever you feel like (assuming she is either 18 years or older, or legally emancipated), but you want her children to stay with you. There aren't a lot of good options in a case like this. Here are a few of them:
1) Avoid calling in outside authorities and convince your daughter to relinquish custody to you voluntarily, or at least until she can get herself straightened out. This may be difficult to convince her to do.
2) Call outside authorities, Child Protective Services, Department of Social Services, whatever exists in your state and report what you have found. Keep in mind, this does not mean you will get custody, it just removes the children from possibly dangerous situations until custody can be determined. This could mean foster homes, or the court might award them to you temporarily. Reporting the illegal things you've found in your daughter's room to warrant bringing in the authorities, though, could lead to her arrest, or depending on the laws and what the illegal thing is, problems for you as the homeowner. Ultimately you are responsible for everything that is in your home.
3) I would talk to your daughter, and to a lawyer. Get expert legal advice from someone who knows your state and the particulars of this case. I am not an attorney, nor that I know of, is anyone on eNotes, so take these suggestions with a shaker of salt and get thee to a professional. I wish you the very best in a difficult situation.
First, let me say I am a lawyer. I have practiced in the family court arena for a number of years, but no longer do so. While this was once my area of expertise, it no longer is, so take my answer with that grain of salt.
The quick, short answer is go to a lawyer that deals primarily in family court law, and do so quickly. If you can get your daughter to agree to go with you and relinquish custody, then that would be the easiest way. Most states will require that custody be signed over in a court order to be legally binding, so filing an action in family court would be required.
If your daughter will not sign over legal custody to you, then you will face more of an uphill battle. If the father is still in the picture, he would have priority in a custody case before a grandparent. This is not fair in many instances, but biology wins the day in a lot of cases. Assuming the father is even alive, regardless of his involvement, he will most likely have to be given an opportunity to participate in any custody case that deals with his biological children.
I agree with the previous poster who said that involving social services is a last course of action. While you may get the children on a temporary basis, most social service agencies are primarily concerned with reuniting children with parents, which can sometimes fly in the face of doing what is in the best interest of the children.
You will most likely not find a short, quick answer to this custody situation, unless all parties involved agree that you are the best person to have custody and agree to sign legal documents to that effect. The best course of action for you is to seek legal advice as soon as possible. If you are unsure of who to hire in your area, the website martindale.com is a great resource. The lawyers there are reviewed and rated by their peers. An AV rating is best, and BV is excellent as well. I would not recommend hiring anyone with a lower rating.
Best of luck to you and your grandchildren!