My children were all well into adulthood when I transitioned. But I do have grandchildren.
Two of them have no idea that I've transitioned because their father, my eldest son, hasn't allowed them to see me in the last three years (three years in July).
My youngest grandchild will never know "him" having been born after I legally transitioned and am full time as myself. My youngest son was initially hostile but we're reconciling, he's introduced me to his son, now 18 months, just this last Christmas and he wants me involved in his child's life.
My daughter has been supportive and has told her oldest child, my oldest granddaughter, who is 15. She was initially a bit confused and skeptical. Then we three went to dinner together, she discovered I liked many of the same things she does such as fashion, makeup, etc., and we bonded very well.
My daughter has not told her 10 year old son, even though she's pretty sure he will handle it fine, but because my eldest son is semi-blackmailing her. My oldest grandchild is close friends with my eldest son's oldest daughter, who is about 9 months younger than her. He has threatened to cut them off as friends if his kids are ever even told that I am transgender and have transitioned. The 15 year old is capable of keeping a secret, hence why she was told. The 10 year old is a typical young boy who just says whatever comes to his mouth.
Hence not telling him.
I understand the quandary in which she finds herself and I do not object to the choices she has made. She has also said that this too will end eventually, since my eldest son and his wife have been treating her and her husband worse over time as well. (The eldest son is a rigid fundamentalist southern baptist who rails on about "sin" but conveniently forgets some of his own actions over the years.)
Additionally, two of my close friends had a son who was not quite three years old when I began transitioning. He's approaching six now and has seen me go through all of this, both presenting as "him" and as "her". His parents have been absolutely supportive and accepting of me, and like most young kids, he took his cues from them. Since it wasn't a big deal to his mother and father, it wasn't to him. This resulted in the following exchange (paraphrasing from memory since it was about a year and a half ago):
Mother: Liz is coming over for dinner. Do you remember Liz?
Child: I remember Liz! I like Liz! Sometimes Liz is Liz and sometimes Liz is David.
Mother: Does it bother you that sometimes Liz is David?
Child: *looks at his mom like she just asked something crazy and goes back to playing*
She was happy at that, and told me that she thinks her son is perfectly content with me, and is growing up where transition is just something people do if they need to.
In the long run, most children will take cues from the adults around them. If the adults are condemning, fearful, upset, the children will be too. If the adults are accepting, loving, and supportive, the children will be too.
That's not an absolute rule, of course, but it describes the majority of cases very well.
