In the case I know of a child who rejected the conversion, the family was quite frum.
Yichud is not always an issue with adoption. When we adopted, we were told it was not an issue for us. Unfortunately, I didn’t ask for an explanation, and I can no longer ask that posek for one.
I know of a situation where a woman with children converted and married a Jew. Her non-Jewish children lived with them. The couple went on to have children, and the non-Jewish children were not a good influence on the Jewish ones.