One of the things bad and bumpy times do besides test us and hurt us, is teach us how to be healers. As we find how to heal ourselves, we learn how to heal others, too.
For your daughter, Mary has it right. Asking the child to keep the secret hurts much longer, and harms more; pretending everything is normal is much worse than speaking openly and gently about the problem. I know.
And your mother -- ask, "Is this about me?" I always have to ask this question about my mother. My life lesson from her is "Don't take this personally".

"What though the tempest loudly roar
I hear the truth, it liveth;
What though the darkness round me close
Songs in the night it giveth.
No storm can shake my inmost calm
While to this rock I'm clinging --
Since love is lord of heaven and earth,
How can I keep from singing?

Blessings,
Barbara