It sounds like your son/daughter doesn’t know how to take responsibility for their own actions and is using you as the scapegoat for everything wrong in their life. I would strongly recommend that you stop helping them because it just keeps the situation going. Why should your child grow up when they have you to turn to with their problems. It’s called “enabling” and you need to stop.
Your child knows they have ADHD – are they taking medication for it? Do they have a support group or counsellor to talk to? They are using your guilt against you. You had no idea there was something called ADHD and the school system and medical professionals didn’t know much about it 20 years ago and kids were frequently labeled “difficult, or a daydreamer, etc. You have no reason to feel guilty and it’s harming you and your relationship with your child. Stop the guilt – it’s a useless emotion.
If it were me, I would do this: stop helping them out in any way (I know it’s hard because of the grandchildren, but if you can get their parents to step up and get the help they need, then it’s worth it). Tell your child that they need to get medicated and find a counsellor and until they do, you can’t help them anymore because it’s having a negative effect on you. However, if they are actively trying to change things and get help, you will absolutely support them. If you want to keep helping your grandchildren and can do it in a way that doesn’t enable negative behaviour in your child, then do so.
There is something called “loving detachment” where you show that you love them, but aren’t going to clean up their messes. If your child never has to face the results of their negative actions, they won’t see a need to change.
I hope some of this makes sense. The other thing I would recommend doing is read up on ADHD so you are properly educated about the condition. Knowledge is power and it will help you with knowing what to expect and what to expect your child to be able to do to help themselves. Your child should educate themselves as well.
ADHD is not an excuse for bad behaviour or ditching your responsibilities. You would also benefit by having a support group to talk to.
Here is a website I use frequently and you may find it helpful. You can find answers about behaviours and lots of helpful advice. http://www.empoweringparents.com
Hope any of this is helpful!