For things like talk events, it would be simple enough to have multiple copies of the same conversation with different portraits based on the parents of the child. It would be kind of annoying and tedious to set up for every conversation, but it’s not too tough.
FEBuilder does have a patch that lets you set events within support conversations which could let you get around this issue, but I’m not entirely sure how it works.
From the looks of it, you’d need to have the “Get Support Level” patch as well to see what level of support the characters have at the very start of the event. Then, you’d branch off depending on what Global Flag is set (Based on their parents, of course).
The real headache comes from supporting two non-related child units together. Even if two children only have 4 total parent combinations, you would need to have 16 conversations per support level. That’s 48 conversations for 2 characters.
Although, thinking about it, none of that really matters since I don’t believe death quotes change even with the “Switch portrait” patch. No matter who the parents are, the unit would have the same portrait while dying anyways. Unless there’s some way to get around that, the “Switch portrait” patch will always have problems for playable units.