As an adopted person of (what I always felt was too much) older parents - in their middle 40s when I was an infant - this simply strikes me as being very weird... I thought it was weird the first two times and it's getting...well...weirder.
My first (and most lasting) impression was that this almost seems to be an attempt to devalue the significance and importance of Brian's biological kids, at least from perhaps a psychological stand point. Anyway it seems that there is some kind of odd angle working here, and personally if your kid's kids are younger than your kids then, Houston, we have a problem.
Brian and Melinda as foster parents I might have been able to see - adoptive parents, not so much. That said, it is nice that Brian smiles these days - something he never did from, oh I don't know... 1970 to about 1995.... something like that. If the kids have anything to do with that then I am luke-warm to the idea... but I would have preferred seeing him improve his relationship with his adult daughters. Maybe it's just me.