Just enjoy your time with them. Take as many "mental" photograghs as you can. Kiss and hug as much as you can. The time flies by faster than you can imagine.
It's strange. My SIL was always saying she couldn't wait until her two kids were 18 and out of the house. She'd even say that in front of them. And she loves it without them in the house. I, on the other hand, didn't want my three to leave. There has to be a happy medium, I think. My problem was that my very being revolved around my kids. When they moved out, and one got married, I went through a terrible time because, a) I wasn't "needed" anymore (which is silly, because they still need me), b) I felt I lost my identity as a mother (does that even make sense?), c) I was going to have to get used to the quiet. So, I did go through a rough empty nest syndrome, but I lived through it and really do enjoy the extra time with my DH. But still.........the best times in my life are when all the kids are home together and there's lots of laughter and constant chatter. Now I have a 15 month old granddaughter that I can cuddle and kiss and play with. That never fails to cause a rush of memories that were so very good.....hehehehe and I can give her back!!!!