My son (5yo at the time) would never have forgiven me if I hadn't told him when his grandmother died. I'd also consider the possibility, in the situation described above, that 24hrs wasn't enough time for the children to process (as best children can) grandmother's death and thus the sobbing.
In my experience, keeping it low key and keeping it hidden are two very different things.
I'm sorry for the loss your family is experiencing. Gentle hugs.....