"How can something come from nothing, as per Big Bang theory?"

There are two possible rebuttals:

• That is not what Big Bang theory says. Big Bang theory says only that the universe started from a point, which expanded outwards and is continuing to expand today (note that this last point is an observation, not a theory).
  
  At no point does the theory state that this point came from nothing; the point could have existed for all time (in fact, it must have existed for all time according to Einstein's theory of relativity because space and time are related, and the original point therefore contained all of both).
  
  Of course, the concept of spacetime may be too complex or alien for younger or less-educated readers to grasp, so perhaps it would be wiser to simply note that the mere possibility of the point existing for all time negates the argument that Big Bang theory calls for it to appear out of nothing.
  
  
• Alternatively, one can simply point out that the theory of evolution does not begin to take effect until the Earth has already been formed, so it is a red-herring. However, in my experience, creationists simply refuse to admit that cosmology is irrelevant to evolution theory no matter how many times you explain it to them. They honestly believe that all of the world's cosmologists, astronomers, geologists, and nuclear phycisists have tailored and deliberately misrepresented their conclusions for the past 300 years for the sole purpose of supporting evolution theory. To say that this is one of the wildest conspiracy theories in history would be an understatement.

The latter approach is simpler, but more difficult because they will often fight tooth and nail to change the subject from evolution to cosmology. And it is difficult to stop them because if you try to stay on the original subject, they will generally make a public spectacle of their victory dances when they declare that you are afraid to debate the new subject. It may help to ask why they're so determined to avoid discussing evolution theory directly, and turn their own tactics against them by accusing them of running from a fight, but their simple-minded distortions of cosmology are so infuriatingly ignorant that it is difficult to resist defending the scientific community on the subject.

Of course, it's rather obvious that their objective is to "prove" that the universe cannot be old enough for evolution to work, but the interesting thing is that even if you could somehow refute the idea of universal expansion, this would not prove that the universe is too young for evolution to be feasible. The sheer scale of the universe and the speed of light necessitates an ancient universe even without Big Bang theory, so this really is a wild goose chase on their part. Even the potential energy released by the gravitational collapse of a proto-planetary matter into the Earth (an amount equal to roughly 3,800,000,000,000,000,000 times the Hiroshima bomb) is so great that the time required simply to cool to a solid state would handily exceed the creationist timeframe by orders of magnitude.

Fallacy watch:

• Straw-man fallacy (pretends that cosmology makes a prediction which it does not actually make, in order to knock it down for rhetorical points).

Recommended further reading:

• Astronomical Society article: An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time.