I'm disappointed that this remains the pathetic reality but I'm not surprised nor confused. This has been the way it has always worked. Humans hold different levels of value to different people depending on which group or locale they belong to. We associate with the journalist more than the Yemeni children so it makes bigger news. The rest propagates outwards from there.
Though I share your frustration, another relevant fact is that the journalist was a Saudi citizen.
Nations get into armed conflicts fairly frequently, unfortunantly. Since the West is also partial to engaging in armed conflicts, I think we cannot so easily critize others without opening a can of worms most would rather not think about: what is the moral difference between Saudi and US bombing in Yemen?.
On the other hand, even at our worst (Guantanamo?), the US is not kidnapping our own citizens and dismembering them.
The Saudis have been bombing Yemen for the last ~3 years and nothing really happened. Then they assassinate a journalist and all hell breaks loose?