"A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on knowledge that has been repeatedly confirmed through observation and experimentation."
It's very important to teach your kids the difference between a scientific theory and a guess. Again, don't let your kids grow up science illiterate. The misconception of a theory usually comes into play on more "controversial" issues such as evolution and big bang theory. Yes, they are theories, but they are scientific theories. Neither is a guess. Evolution and big bang are theories developed and based on an unbelievably large amount of scientific data/evidence.
Claiming that creationism is just as valid as evolution and big bang because they are all theories, is just plain false. Creationism is not a scientific theory. It's not based on evidence (unless you want to include the Bible as evidence, at which point we have a completely different topic to discuss!). Evolution and Big Bang Theory are.
Here's something else to chew on. If Evolution and Big Bang Theory are just theories and shouldn't be trusted, what's that say about the Theory of Gravity? Do we no longer trust/believe in gravity?
This post isn't intended to argue evolution vs. creationism. That debate was settled by science long ago. The point of this post is to look at the misconception that most people have with a scientific theory. A scientific theory is NOT a guess. It's based on data/evidence that has been analyzed in many different ways. Teach your kids science literacy by teaching them the correct definition of "theory".