I just had another thought: I think all the "God wouldn't ask me to do that"-type retorts to the OP have been based on the assumption that you were being asked to do it as a sacrifice for the remission of sin... but the Bible's full of stories of God commanding all sorts of strange things for other reasons:
"Hey, Joseph! Grab Mary and Jesus and head for Egypt *right now*. Trust me."
"Hey, Lot! Whatever you do, don't look back at Sodom as you leave. Take my word for it - this is important."
"Hey, Moses! I need you to get everyone to paint their door frames with lamb's blood... and you need to have them do it before nightfall. You'll understand why later."
Why would it be out-of-keeping for God to say "Hey, _____! I need you to kill your kid. Some day you'll understand why, but for now you'll just have to trust me: I REALLY need you to do this"?
Maybe it's for remission of sin; maybe God has other reasons... but how can you exclude the possibility that in all his mysterious ways, God's good, perfect plan is best served by you stabbing your child to death if he says that it is?