1. In order to make moral decisions, humans are required to analyse situations
2. Situational analysis requires sensory information
3. Sensory information is by definition subjective
4. Therefore, morality has a subjective component
5. Therefore, morality is not objective.
If you don't like sensory information and feel like you can use pure logic instead, try this one
1. In order to make moral decisions, humans are required to logically analyse situations
2. Logical decision making relies on information
3. Access to information is contingent on the subjective experiences of a given human
4. Therefore, moral decision making is subjective.
Don't get me wrong, I am not arguing for an "anything goes" system. You can make a decision based on as much logic, facts, and empathy as you want. I encourage that. To me, that seems the closest we can get to objective morality. Also keep in mind subjective morality does NOT mean arbitrary morality (this is a common mistake religious people make when arguing against "human" morality). Just because I need to rely on my experience and my decision making doesn't mean that murder being wrong is suddenly a matter of opinion, even if whether or not a given situation counts as murder might be a lot less clear cut.
This is also why we don't hold mentally handicapped people to the same moral standards as everyone else. If morality was objective, what they do would be equally as moral or immoral as everyone else. But because they are largely incapable of moral decision making, they cannot be considered immoral. This also makes "moral" the default position - everything is moral until it can be reasonably inferred that it was done with ill-intent or was designed to cause harm, then it becomes immoral. I'm fine with that. Things like negligence are a lot less clear cut.
This is why I consider "Objective" vs "Subjective" to be largely meaningless. I don't like/use the terms because everything we experience (and thus reason about) is inherently subjective, making it largely ill-defined. A moral system based on personal experience that attempts to analyse situations and apply empathy is subjective, as is a completely arbitrary system where people do what they feel like, which to me undermines the value of the Objectivity/Subjectivity debate, because "subjective" can mean anything from well reasoned moral positions based on experience to complete anarchy. I much prefer to define a ruleset, such as "give everyone the maximum freedom unless it can be shown to negatively affect others", even if it's arbitrary, to analyse things within using pure logic, then everything inside that box becomes """objective""". It's not a strict definition, and makes philosophers cry, but it's useful, and IMO is the closest we can get to objectivity.
Also consider, if morality is objective, we should be able to inherently measure the "moralityness" of something. In a universe with 1 human, everything is inherently moral, so this "moralityness" doesn't exist.
"Objective morality", in fact morality itself, is a luxury of living in a healthy society. When people are put in situations where they have to do bad things to survive, morality becomes a lot more fluid, as people have to judge degrees of harm from a collection of bad actions, rather than good vs bad options. Morality itself seems to be a series of rules we effectively made up as a collective society for the benefit of everyone. I don't think there's anything special underlying that. It's good that we have these rules, but I'm not going to pretend we are tapping into some inherent aspect of the universe or anything.
Also, OP, I disagree with your first premise. Young children, psychopaths, and mentally deficient people are often not aware of the fact that they can commit evil. Even if they are aware of that high-level fact, they may not be able to determine if a given action is evil or not. The second premise suffers from the same problem. The argument is also a non-sequitor. What does the existence of universal, physical laws have to do with objective morality?
Also, the debate around abortion is inherently one about whether or not it's right/wrong. Determining whether it's murder is also determining whether it's right or wrong, because by being murder it is inherently wrong.