Sedna at perihelion is 76 AU from the Sun, meaning it receives about 1/76^2 = 1/5776 as much light from the Sun as Earth does. This distance reduces the sun from a -26.7 apparent magnitude to a -17.3 apparent magnitude. The Moon has an apparent magnitude of -12.6, so a quick answer would be: the Sun illuminates Sedna at perihelion about 100 times as much as a full moon illuminates the Earth. The human eye isn't linear, so 100 times as light doesn't mean "looks 100 times as bright", but the directional answer seems clear: people can see enough to get around by moonlight, so they should be able to see better than that — more like twilight than moonlight, and enough for some color vision. Compared to either the Sun or moon as seen from Earth, the Sun as seen from Sedna will be much smaller (less than 1 minute of arc in diameter), so it will cast much more sharply-defined shadows.
At Sedna's projected aphelion of 936 AU, the Sun drops to an apparent magnitude of -11.9 (a little more than one millionth of its brightness as seen from Earth, and one-half the brightness of a full Moon). Visibility is still possible under these conditions, but limited. The sun would have no apparent size; it would just be an extremely bright star.
As for any hypothetical moons of Sedna, it's hard to guess about their appearance without knowing anything about their size, orbit, or composition. I'll leave that one alone.