The sick child is SGI and NST

"Answer: Then since I have no choice, I will try to give you a brief explanation. The 'Teacher of the Law' chapter states, '[Since hatred and jealousy toward this sutra abound even when the Thus Come One is in the world], how much more will this be so after his passing?' The 'Life Span' chapter states, 'I will leave this good medicine here.' The 'Distinctions in Benefits' chapter speaks of 'the evil age of the Latter Day of the Law.' The 'Medicine King' chapter says, 'In the last five-hundred-year period you must spread it [the Lotus Sutra] abroad widely throughout Jambudvīpa.' A passage in the Nirvana Sutra reads, 'Suppose that a couple has seven children, one of whom falls ill. Though the parents love all their children equally, they worry most about the sick child.'

With the clear mirror of these passages one can guess the Buddha’s intent. The Buddha did not appear for the sake of those present during the eight years when he preached the Lotus Sutra on Eagle Peak, but for those who would come after him, specifically for people like us, those living in the beginning of the Latter Day, not for those who lived in the two thousand years of the Former and Middle Days. 'The sick child' mentioned in the Nirvana Sutra represents those who slander the Lotus Sutra after the Buddha’s passing. The Buddha will now “leave this good medicine here” especially for those who, the sutra says, 'though the medicine is of excellent color and fragrance, do not perceive it as good." --The True Object of Worhip