There are usually two types of Seniors, those who are ready to be independent and those who could live at home forever. Although you love your children, I am sure that you and they are both ready for them to become adults, whether they realize it or not.
Some ways to help them realize that they are ready is to start setting up expectations for them. Have conversations about what you expect of them when they come home on summer breaks and when they graduate. Ask them what they expect from you while they are at school. For example, do they expect you to pay their tuition and only call once a week, or do they want you to visit them all the time, or never visit them. Talk to them about what they are looking forward to when they get to school, teach them about finances and how to manage their money, and also how to do their laundry. Having these types of conversations will help them get in the right mindset, but it will also set up clear guidelines for both you and them on what to expect during this transitional period.
You can also start to go out and pick up stuff for their college dorm, and discuss how they would like to decorate. Spending some quality time together will also help. One way I did this, is my mom and I went to go see Toy Story 3 when it was in the theaters. This was a perfect bonding moment, because I grew with Andy (the boy in the movie). I was the same age he was when the first movie came out and I was going away to college the same year he was. My mom and I both teared up a little bit while watching, but it helped us both start to understand and begin the transition of me moving out. Finding little things that you can do with your child that will not only be fun and reminiscent, but also informative will help you both get through the transitional period much smoother.