Joseph has rarely dipped out of the top 20 in the past century. It's a classic name that means "God increases," and it's been worn by a number of notable people, from the stepfather of Jesus to the current Vice President Joe Biden.
Frederick is just as classic, though nowhere near as popular here in the U.S. It's a German name that means "peaceful ruler," and is currently just shy of the top 500 names in the U.S. for boys. As for Frederick namesakes, there's Frederick Douglass, a social reformer and abolitionist, numerous European royals (they're up to Frederick IX in Denmark!), Frederick Law Olmsted, the revolutionary landscape designer responsible for creating Central Park and numerous other urban green spaces.
In all, it's a name that's very "old-school," classic and reliable—and it makes a nice counterpoint to the equally classic, though more unique moniker they chose for their oldest, Arabella.
What do you think of Ivanka's baby name choice? Too traditional or just about right?