Knox and Vivienne's Sun and Venus are both hidden in the 8th house, in the tender and protective sign of Cancer, which can make getting to know them on a deeper level takes some time. Even with normally outgoing Sagittarius Rising, they might be a little bit shy with new people as young children. When they get older, they will probably develop that Sagittarian openness and friendliness as they way they naturally greet the world, but the Rising sign is also how we seem to be to others, even if it's not the whole story, so Sagittarius it may make them seem more confident and like they are an open book, when really, they are far deeper than people may suspect.
There may always be a sense of intensity, intrigue, and mystery about them, though, with Pluto in their first house, a planet that carries a lot of gravity, figuratively speaking, considering its small size.
From Knox and Vivienne's Kiddiegram:
Pluto in the First House
Knox & Vivienne can be quite a force with which to be reckoned. They take life and themselves quite seriously and throw themselves into their activities with their whole heart and soul. Their quiet intensity and willfullness may put some people off at times. One thing to remember with Knox & Vivienne is that they do not like pretense. They sniff out hypocrisy or fakeness quite readily, and may not "smile for the camera" when you want them to.