Tell her that Hashem destroyed the Beis Hamikdash bacause of sin’as chinam etc. but the reason he chose that day specifically was to remind us of our mistakes in order that we remember to better ourselves, because that is the purpose of galus.
In general I think this is the approach to be taken with such midrashim, they are not said so much as to tell us historical facts as they are to give us a perspective on our current situation. It’s almost as if the chachamim are giving us mussar, saying “Look, you cried then, and it’s no wonder you’re still crying now, you still haven’t bettered yourselves!”