Enjoy them while you can. Life is short and not as predictable as we'd like to believe.
This is bonding time, as many have pointed out. The grands will love you and have wonderful memories, as will you.
Have had two grandkids live with me for several years, and loved every second of it. I miss them terribly, as their parents moved to other states. They come for a few weeks in summer and we have a great time.