Depends on how active you really are. We've been many times, most recently this summer with a few of the grandchildren. The very first trip with the kids I planned a week. Two days was more than enough. With the kids, when I was young, we hiked a day, saw the exhibits, and drove the rim. Now, a few hours is enough. We look at the splendor from a few spots, had stayed and braved the cold for a spectacular sunset, never tried sunrise, not my thing. So if you like biking, you can easily spend a full day biking the south rim. If you do the I-Max thing you can spend a day with the movie and walking the displays at the South Rim, no too far away. Personally, I would basecamp in Williams, and spend a full day at Bearazona (for Adults and family alike), if the children are young enough a few hours at the DeerPark. You can bike the many trails around Williams or just ride through downtown Williams.
Have a wonderful time.
Happy Trails.