First, I don't think you actually understand Kant or the categorical imperative.
It's late here, so forgive me if this is short. I can expand it later.
Abortion and even in infanticide (if you want to go that far) are dead easy to justify under the "act in the way you would like everyone to act" imperative, as long as you have a reason for what you are doing. In fact the CI is so open to such logic that you can even justify killing adult humans.
If was a repugnant sort of person such that I could say honestly "If I had Down's Syndrome I would like someone to shoot me, thus I shoot people with Down's Syndrome" then under the CI I would be behaving morally because I was behaving in a way that I would genuinely wish everyone to behave.
It would be nice to think that the logic had a gap in it, but exactly the same logic justifies things we tend to find less appalling. Example: "If I was terminally ill and being kept alive in terrible agony by life support, but had absolutely no hope of recovery, and I was unable to communicate my wish to die, I would like to have the plug pulled".
Basically I don't think Kant leads directly to any clear moral path when it comes to abortion, euthanasia and other really tricky problems.
In my case, I don't think that I was particularly valuable when I was a fetus or a very young infant in my own right. If I had died then for whatever reason I think that would be far less tragic than if I died tonight. As long as I'm consistent about it I am right with Kant.
Second, the fact that you have kids is both obvious and relevant. If you had kids, you would realize that equating them with a rock is ridiculous in the extreme. While I appreciate the humour in your drunk example, being drunk impairs your mental functions. Having kids does not. Perhaps you should have some before you decide that they are the equivalent of rocks. Actual first hand knowledge.
No, having kids does impair your judgement. A woman's body dumps hormones a few days after giving birth that make them think their squalling poop-tube is wonderful and precious, and the effect lasts a very long time. Male parents get the same hormone dump. This is why your kids are amazing and wonderful, and you neighbour's kids are ill-trained simians that smell.
Mind you the hormones leave a post-natal dumpster window before they kick in, so that in hard times our ancestors could bin an infant they could not feed. Evolution only wants us to take care of babies if doing so represents an odds-on bet of passing on our genetic payload. If an infant is unlikely to be a good bet evolution wants us to discard it, which is why we find babies in dumpsters every now and then.
The fact is infants don't have a mind or a personality for quite a while after birth, and it's years before they are even as smart as a chimp.
There's an interesting article here about research into differences between chimp and child cognition. Different but comparable.
http://www.nytimes.com/2005/12/13/science/13essa.html
My comment was (mostly) tongue in cheek. But what is irrational about believing that it is wrong to leave an infant in a dumpster because you don't want it any more?
Nothing, it's just not right to think it's
as wrong as abandoning an adult human to die.
At what point do you say that an infant attains some sort of innate moral worth? What scale are you using to calculate this moral worth? Is it a sliding scale - some are worth more than others - or is it more like a switch - once you hit point X, you are equal to everyone else? What is YOUR system of morality, and does it stand up to rational scrutiny?
If you can carry on a conversation, even a very primitive one, you're in the human club.
If you can communicate by stringing symbols together without syntax you're in the chimp club, and I personally think chimps should get the same rights and respect as mentally disabled humans of comparable intellect and physical capabilties: I am strongly against using them for medical research, for example. Humans get here fairly quickly.
If you don't have a concept of a future or past beyond instinctive behaviours but are capable of solving simple problems and whatnot you are in the horse/cow/dog/rat club, and while cruelty to you is intolerable it's not an unforgivable crime in my mind to kill such entities in a humane fashion if there is good reason to do so.
If you're essentially a biological automaton like a wasp or an ant, or you are a plant, you don't get no respect in your own right. You may well be beautiful or useful or a part of the ecosystem but I don't care either way about your welfare for its own sake the way I care about a human's welfare for its own sake. I don't lose any sleep because birds eat insects and cows eat grass.
A newborn human is such a crappy piece of protoplasm by any objective standards that it has to go somewhere between grass and a rat. It's got more going on than lettuce does but a whole lot less going on than someone's pet rat.