I have a diary my maternal grandmother kept just after she was widowed, so that my aunt and mother who were toddlers at the time would have some of her memories of their father. Much later (when I was 13) and she knew she was dying of cancer, she wrote another diary telling her daughters why she had decided against treatment.
Both my mother and aunt found these diaries painful to read immediately after my grandmother's death, but later they were glad they had them, as were my sister and I.
So my advice is: keep them for your daughters, but go through them now, on your own, and write a letter explaining anything that might hurt one or other of your girls.
Alternatively, you could tell your daughters these diaries exist and suggest they read them now while they can still come and ask you about anything that they want to know more about. I know my mother would have welcomed the opportunity to ask her mother about one or two things.