In and of itself the alt-right would, I believe, prefer that illegal alien adults be separated from their children. There are many good reasons to do so.
However, in the larger scheme of things, keeping families together serves our nationalist interests better.
It at least partially takes an issue off the table for the 2018 elections. It also gives Trump the humanitarian glow he’s been lacking the last few days as he’s been under attack as the new Hitler.
It’s more likely that funding for the wall will be secured too. Likewise, at some point smart, reasonable people are going to wake up and say, “Hey, let’s keep the families together permanently by deporting them very, very quickly after they cross the border.”