If she's a toddler you have plenty of time to figure out what you wanto tell her.
A toddler might want an answer for appeasement where as a tween might ask a probing question. Then it depends on the context of the question. What is she looking for? Did something come up at school or a friend say something or geez there's so much to think about.

But you have time & you know her best. Yes honesty is best, but honesty can also be brutal.