If it was implemented well, I'd have no problems with it. I would like to have the option of adopting children of different races and not just human children. Would like to see what a dwarven or Qunari child looks like.
Maybe it could be a discussion with your LI beforehand? Or maybe your LI could react to it differently depending on whether or not you discussed it with them? That would make it even more realistic, imo.
If they include that maybe an LI not liking kids at all...(sorry, people wo don't have and don't want children might be a minority out there, but we still exist, so a nod to us (I am such a person) would be nice)
As for the option itself - allow us to train the kid (or not - if you don't want to)...like an apprentice (either you do it yourself...or let one of your companions/advisors do it)...at least in a future game (it's too late now IMHO)
Said apprentice would be your second-PC-character (so you can basically play two classes at once (more or less) if you want to - or just one, in order to have a spare *add whatever class you like*)...the sex of the child could be decided by being given the option to rescue one of two children, either the boy or the girl (race and looks could be something you get to chose...), you could send them on missions, too (note: child is a little wrong - I am talking about a character around 15-17 years of age (at the start of the game)...so teenager is more fitting) - those missions would happen at the same time (at least the game would state they do, of course you can't actually lead two parties at once...though switching between two parties might be nice for certain missions, if say the PC is showing up at a ball to mingle with the nobles, forge connections etc. (and act as a distraction for the second team) while the second team is there to steal something out of the vault or free prisoners or poison the food or something...this might be a nice story telling tool (as long as it's not over-used - so not every mission should work like that)) as the one your "real" PC is undertaking....well, just an idea and as I already said:
Next time - maybe? (too late for DA:I)
greetings LAX