From your own narration, your employer seems to have acted with much reasonableness and consideration by granting statutory maternity benefit and all the leave so far as you required in the wake of your delivery. We cannot expect an employer to be constantly considerate according to the personal needs of an employee howsoever genuine they might be.
Working women's work -life balance becomes all the more difficult with child delivery. Some women take the bold decision of job/career relinquishment for the sake of the new born children. It is, Of course, a matter of individual financial position.
Therefore, you can request the waiver of the notice condition in view of the costly treatment being given to the baby as a gesture of humanitarian consideration from the end of the management. Alternatively, You can request the management to keep the formal acceptance of your resignation in abeyance for some more time so that you may withdraw your resignation if the baby recovers in the meantime.