Are babies innocent or sinners?

After writing my previous post 'No meaning of life for atheist?', I came to think if Christians are right why babies are not born straight to heaven.

In short, according to Christians, the life on Earth is just a testing period to be decided whether to live on Heaven for eternity or live (if you can call that living) in Hell for eternity, right? And according to the all antiabortionists (most of them, Christians), babies are innocent, right? Do you not think the two ideas collide? 

If babies are innocent and get good or bad in this short life on Earth, why just let them born to Heaven? Original Sin, you say? OK, that means babies ARE NOT innocent, but sinners. Aborted  babies will go to Hell, because of Original Sin? That sound absurd  now even to most of Christians (save that Christians used to believed so). So I heard that they now have fabricated a theory that child under certain age cannot go to Hell no matter what they do.

Hmm, interesting. I heard that the 'age of innocence' varies among Christians, just like everything about Christianity. Say it is 7. That means that God is not offended by the things that a 6-year old did. Was the intellectual ability of Adam any better than that 6-year old? He was just born (even though he had the body of an adult), he had no ability to tell good from evil, he had no education whatsoever. What the hell is the reason that God could be offended by him not following a simple order? Why the all-powerful, all-knowing, all-merciful, perfect-loving God cannot just forgive Adam? Did he not suffer enough afterwards?

Christianity surely is a mysterious group of ideas, where perfect-love and conditional forgiveness coexist.