If you thought that your father was doing drugs, what would you do?

You do need to confront your father, but not about drugs. Confront him about the lack of relationship he has with you and his grandchildren, and his overall behavior. It appears this issue has been going on for some time, and it may be because of drugs. Tell your dad that your hurt and really bothered that your not as close as you should be, and you want his grand kids to know him better than they do. If you want to help him with any other problems he may have you need to have a better relation with him. I don't know if your dad knows you feel he behaves childish or not, but if he does and is taking drugs too, he may feel more childish than ever, and has a hard time facing you, or getting close. If you want a relationship with your dad it's probably up to you to make it happen. If he is on drugs, he needs to feel important to you, and loved by you, before you can talk to him about any other issues. If you do all you can to fix the relationship, and he doesn't respond you may have to let it go, and hope he realizes the time he's loosing with you and his grandchildren. If you confront him about drugs, and have a bad relationship with him, it will only push him farther away.