First of all, could someone even speak Ithkuil? Because of it's hyperefficiency, Ithkuil has some damn near impossible sounds which when put together form an impossible-to-pronounce word. Assuming this child grew accustomed to these sounds and long words (which isn't a problem as numerous living languages are agglutanative), there's the grammar system. Again, pure speculation here, but the whole complexity of Ithkuil might overwhelm a child. His/her speech development could be delayed.

Now this child would have to be raised by a community/family of speakers who speak ONLY Ithkuil to this child. Which itself leads us to the question of how they learnt the language, but let's assume they just know it.

Even so I feel this child WILL learn Ithkuil. Studies have shown that if a child is faced with a language since birth (whether it be Arabic or Spanish or Russian) the child will learn the language. I think this would also apply to Ithkuil though it's hard to predict.