But we have so many younger characters. They're called the 'young Avengers' for goodness' sake. And we have the Champions too.
It could make sense if Vision didn't think those 2 were his actual sons, with the whole alternate reality and reincarnation business from the comics. It would be a little awkward when your twin sons were separately raised by two separate families. With Vision currently emotionless (or so it seems) in the MCU, maybe he won't regain his attachment to Billy & Tommy if & when they reunite
I can't be sure. Maybe this is one of Cerebro's secret abilities?