I disagree that giving someone ethical CONSIDERATION should depend on that someone's capacity to behave ethically. Infant humans are incapable of ethical behavior. I don't think that this justifies using infants in any manner that you wish regardless of the interests of that infant. Are you suggesting that severely mentally or emotionally undeveloped humans are not deserving of ethical consideration? If so, then why not USE them? (subject them to experimentation, harvest their organs, milk them, enslave them, eat them?)
Capability does play a role in moral consideration, but not in the way that you suggest. Rights are granted (by those capable of advanced reasoning and ethical behavior) to someone based on their ability to exercise those rights. Animals are granted a right not to SUFFER unnecessarily because they are capable of SUFFERING. Human animals are granted the right to free SPEECH because they are capable of SPEAKING.
I agree that we should not be unnecessarily cruel to animals. How are you defining necessary?