My philosophy on second opinions is, if you think you may need one, get it.
Don't hesitate. It is always good to get another perspective on things.
First of all let me state that it's great that the hip dysplasia was recognized and diagnosed. I have seen a few adults that suffer from the same problem and were never diagnosed as a child. The statement from the doctor about being continuously monitored yet the follow up is in 8 months does seem a bit contradictory. You may want to ask him to clarify this point.
I hope your daughter improves.