I think, if Mikoto had a child headmate, Mikoto would be a good caregiver since he seems to be a good brother to his little sister, but it would take a heavy toll on his mental health. He definitely works too hard to please and care for others, and he would get stressed out by the responsibility, so I think if their little was to have a caregiver, it would have to be someone outside their system.
That being said, I think John would struggle with the task of caring for a little, but ultimately could do it reasonably well. It would be a steep learning curve, and he would definitely use language that wasn't child-friendly – either by accident or because it wouldn't occur to him to censor himself around a child – but given how eager he is to protect and look after Mikoto, I think he would be willing to give it a try.
