When asteroids crash into the Moon they give of huge amounts of energy resulting in a "bright" flash, as can be seen in this video. At the end of the video its also says that anyone looking at the Moon at that moment would have been able to observe it without a telescope.
But what kind of collision would be required to be able to be seen from Earth when it would involve more distant bodies? Would one of Jupiter's retrograde moons colliding with one of its prograde moons be visible?
A real example of a high energy collision would be the collisions of the fragments of comet Shoemaker-Levy-9 with Jupiter. However according to this video from ESA it happened on the opposite side of Jupiter, so not in our view.