(These things are always so difficult to imagine. How do we really know what we would do?)
I think I would set aside that time and try to create my own acknowledgment of what the whole thing represents by going someplace quiet and lovely like my room or a garden spot or a park. I think I would sing a hymn either in my head or out loud to commemorate the occasion and I probably would take my bible and kiss it lovingly, sending that kiss to the child's cheek. I might also take a pad of paper and write about how much my caring means to me and offer it up with my prayers.
The only experience I have had that might relate to yours is when my son's father was dying. He was on life support and my children and grandchildren had gathered around him. His second wife told my kids that I was welcome if I wanted to come. (Which I think was amazing.) I thought about that, and I realized that I was really an outsider and that they all had enough to deal with when they ended life support without having to cope with the dynamics of my grief (and probably guilt.)
So, I went to my little retreat and did my best to "spend some time with him." I sang and prayed and wrote. Later, when I attended the funeral, it felt complete to me.
The circumstances are nothing alike but having a "surrogate-service" might be worth considering.
Blessings, Luise