You love them. You nourished and supported them. You sent them out into the world. Now it's time for your children to nourish and support themselves. Setting boundaries for your adult children is important for your relationship but can be difficult, especially if those adult children are struggling.
Keep your housekey. When they left home and you told them they'd always be welcomed back, you didn't mean it literally. You're entitled to the courtesy of a phone call before your adult children come to visit. Let them know that if they're in crisis, they're welcome to call on their way to your house, but they shouldn't have a key or come and go as they please. Setting boundaries with your adult children means expecting common courtesies.
Listen. It's one of the greatest gifts you can give your children when they become adults. Listen to their stories and their troubles, and let your children know you've heard them. Then, ask what they plan to do next. Listen without judgment or action. You're not responsible for fixing your adult children's problems. Set your boundaries so that you don't lose sleep figuring out how to fix their problems.
Send the grandchildren home. Visiting with your grandchildren is pleasant. It can be fun to develop a relationship with children who are not your own but share your heritage. Do not to take on a parenting role--requiring your adult children to parent their own children is one way of making your boundaries clear.
Hold on to your checkbook. Financial boundaries are important. One thing that makes children adults is their ability to provide for themselves. Everyone makes bad choices sometimes, but if they don't have to live with the consequences of those mistakes, they'll never learn. When your adult children are in financial trouble, bailing them out should be a last resort. If it comes to that, charge a fair amount of interest and require them to pay back the loan.