"I don't know, but let's look it up" is an awesome answer!

It teaches the kid what their resources are, and gives them a handle on how to look stuff up independently in the future.

Plus, if you're going with (shudder!) Wikipedia, it means there's an adult to translate the ridiculously obtuse language that Wikipedia uses for all things science:

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields.[12][13] It has a diameter of about 1,392,684 km,[5] about 109 times that of Earth, and its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System.[14] Chemically, about three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. The remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.[15] - http://en.wikipedia.org/wiki/Sun

Any parent who STARTS with that is probably not helping any more than Calvin's dad explaining "Old photographs are black and white because the world didn't gain color until sometime in the 1920s" :P