Maybe not from a halakhic or legal standpoint, but possibly yes from an ethical one. That is because a child has more life ahead that he could realize; in other words, more life was taken from him. Besides that, it is more severe in terms of the cruelty involved in the act, since the child bears no responsibility whatsoever.

Papagayo
One could also say that an adult has more value because he has more memories and therefore more personality, awareness, and understanding. This is a bit subtle so maybe it sounds like a strange argument, but let’s sharpen it by taking an argument by analogy from the abortion debate: “A fetus has no personality and therefore has no moral value” (as for awareness, there is fairly broad consensus that even a one-year-old baby still does not yet have awareness).
I think one can also feel intuitively that when the life of a man is taken, more life is being cut off than in the case of a baby.
Of course, I do not really think an adult’s life has more value, but rather that it is all emotions and there is no ethical difference between an adult and a child.

Does “collective punishment” apply more absolutely in the case of an adult? (If so, that would make the intuition that people are more horrified by the killing of children understandable.)

Ezra
Your wording about collective punishment applies only in very specific cases… Your point could be put more simply: children are usually more innocent, and so it pains us more when a symbol of innocence is killed.
From the standpoint of vandalism, killing an adult is much greater vandalism; it is the destruction of a whole enterprise and of many years of mental and physical activity (just think of the long and exhausting process of building a personality, experiences, and aspirations). The adult’s interactions with the world are far more extensive than a baby’s, and the impact on the surroundings is broader as well.
Again, one can argue a lot in either direction, and I do not think we can reach a decisive conclusion (not that there is no truth, but it is beyond the reach of our intellect).