Chomping at the bit is what causes some of the problems. It's very easy to find yourself thinking "its ok, I can do a little running, it's only kids" I can understand that perspective, and I even pushed it a little early and probably should have waited a few more months. Anyway, heed everyone's advice, wait the 6 months ot a year, or better yet closer to a year. I don't there are many surgeons who will tell you to that its ok earlier than 6 months. Also, when you look back, the year is actually pretty quick (I'm coming up on my first annual post-op appointmen) looking forward to bending the doc's ear.
Along the lines of Jerry's post regarding bone growth, if you take a look at a series of consecutive x-rays, post-up to a year or more, and hone in on the femoral neck, you'll see some pretty dramatic changes to the shape of the femoral neck over that period. It actually thins a bit, which seems counterintuitive, still strong, but the forces are slightly different and there was bone loss during surgery, so it's all reworking itself to become strong again. All I'm getting at is that even after one year, you'll still see slight changes in the shape of the femoral neck meaning that the femur is still adjusting. I haven't looked closely at the acetabular side, but I think the same thing happens there. In short, bone growth is slow.
Also, there is quite a bit of info on the subject here. I remember seing a pretty good interview Pat had with Brooks, or Mont (can't quite rememeber), talked a lot about bone growth post surgery.