In fact it is a positive disadvantage in many situations - for instance if you have to play a piano which, though perfectly in tune with itself, is allover flat. If you had perfect pitch, this would drive you mad, which it shouldn't, because present-day "concert pitch", although useful, is only an arbitrary thing, not a requirement for good music-making.
Supposing the child in question decides to take up a baroque instrument: that would entail not only re-learning at lower pitch, but readjusting the position of some of the notes. "Perfect pitch" is the enemy of musical flexibility.
What musicians, including children, need is to be able to reproduce good "relative pitch" at an early age, ie to learn the skill of making the second note accurately and naturally in tune with the one before, and so on.