That is a terrible event to endure. I am sorry to hear of such tragedy for the parents and rest of the family. You don't say whether that family is also 'Christian' or a different belief system. If 'Christian', maybe you could pick and choose as you feel most fitting from Baker Funeral Handbook. Pages 123-137 may be helpful.
I do not know of the "homily section". I would know it as the acknowledgement of pain of the survivors, the remembrances and love of the deceased, and the attempt to comfort the survivors. If entire service is not done at graveside, then there may be a small honorarium said graveside later. The ritual options are described in the Baker book, or organize your own based on your personal knowledge of the family.
Or you could say what you actually *feel* and with your own empathy, know what anyone in that family's situation would feel. Acknowledge the tragedy and pain of the loss of (Deceased Child's Name) and the ongoing struggles of (Still Living Child's Name). Note that DCN never had the chance to live and grow, but that DCN knew love of family during his/her brief life. She/he knew 'The Human Touch' (author unknown) page 95 of Hutton's Minsters Service Manual or attributed to Spencer Michael Free, with slightly different wording (http://poetryprayersthoughts.blogspot.com/2011/10/human-touch-by-spencer-michael-free.html). She/he will never be forgotten, but will live in family's memories and hearts forever. Acknowledge that SLCN is receiving the best possible medical care; that she/he is loved. Everyone *envisions and believes* that SLCN will recover and come into our home.
Close with comforting poetry maybe like putting Human Touch here instead of the way I suggested for the babies (but I really like it with/for the babies, especially the one who passed away but still felt and gave love in his/her short life), "The Passing of a Friend" by Dorothy Talbott Foster (pg96 Minister's Service Manual) words adapted to brief life of deceased infant. "Lean On The Lord" by Clinton Lockhart (pg 98 MSM) can be adapted to friends, family, the powers of nature or the universe as is appropriate, or used as is for Christian. Add a message of hope for the still living child. and healing peace and love for the family.
HTH, many interruptions and missing facts, not conducive to clarity of thought. You can do this Henry.