Truthfully, the best that anyone is ever going to get is to have an answer that's good enough for them, for the time that they have available. Satisfaction is a good goal to shoot for, especially when it comes to children, who are unique and interesting creatures unto themselves, defying the idea that any sort of solution will work for all of them.
Getting comfortable with the idea that a better answer will arrive right after the person leaves is an important part of building the confidence that will be needed to impart expertise - give the best answer at the time you have, and if something better appears afterward, be ready to tell that if they come back or to the next person that comes in with the same question. Or just mention it in the next week of Story Time.