[The looks Sauron is getting from him are certainly entirely elven in nature and cheek. No simple Uruk would give him such open expressions willingly.]

Why should a child scream? He does not hunger, he is not wet, he is not alone. Those are most of the reasons a child might fuss.

The Uruk children scream because of all of those things, and their [twisted] nature. Elvish children do not. [Perhaps Fëanor had and that was why his mother quit the world but he could not rightly say.]