You have to consider the idea that a baby is now or once was a personality, a will, along with the capacity for pain and suffering. This is why abortion is justifiable, but euthanasia against a comatose patients with a possibility of waking up is not, for example. It violates an already existent will, or personality.
Well abortion harms the human life by destroying it, but I can see your main point was that abortion causes the fetus no pain or suffering. So because the fetus does not feel pain, it is ok to kill it? Say that these young children who were being burned in these rituals had been given sleeping gas or some sort of drug that would prevent them from feeling any pain. According to your logic, the killing would then be justified.
This is true... but a fetus is so drastically different from a child, that any and all reasons in our nature that we would consider it morally wrong to harm or kill a child or a person, simply do not apply. Just ask yourself what the practical reasoning is for why you cannot just murder people that annoy you or inconvenience you. None of those reasons apply to a fetus that has never had a functioning brain.
I know you are going to respond by saying that a child is different than a fetus. But then you could also say that a child is different than an adult. An adult is a fully developed human, and a child is a developing human, where a fetus is just the start of the process of human development. A child has no less worth than an adult, and a fetus should have no less worth than a child. Without a fetus you have no child. Even without bringing religion into this, I cannot see how one can justify abortion.