I must be dense.
No, you are not dense. The entire issue of Adoption is a complex one to enter into TMG. Remember that TMG is primarily designed for tracking genetic relationships, so Adoptions have to be kind of "squeezed" into TMG. From TMG's point of view Adoptions are an event not a relationship, although most people consider an adoptive parent & child as having a relationship.
My on-line book about customizing TMG has a very lengthy and detailed discussion about Adoption which begins here. As that section describes, I find it useful to create several customized tags to deal with this issue. I use custom event tags which have the adopting adults as Principals and the adopted children as Witnesses. Such event tags show in both the Principals' and Witnesses' individual views, so that this situation/event becomes clear in the narrative of all participants. (You do have the "Preferences / Program Options / Tag Box / Show witnessed events" option checked so events show in Witnesses individual views, right?) Since my primary output is Second Site (which will list all relationships, both Primary and others) I also add custom relationship tags when that seems to make sense as described in that discussion.
You may not wish to get as thorough and complex as I do, but reading my thoughts about customizing TMG to record Adoptions in TMG hopefully will give you some useful ideas,