"They don't know yet," she shares. "I want them to hear from me first what Mommy has and that I'm going to be fine. I don't want them to be afraid."
Harris, 40, and her husband of 10 years, financier Michael Hess, made the decision that telling Josselyn, 6½, and Hillary, 3, separately would be best because of their age differences.
And at Hess's suggestion, the couple will make a video diary documenting her health journey and will decide at some point down the line if they'll show it to their daughters.
"The day we found out while it was still fresh in our minds, we made a video," she recalls, adding they have made two more since. "I don't know if we'll end up showing it to them, but at least we have the option."
But for Harris, it's all about keeping things as normal as possible for her family. She tells PEOPLE candidly, "I'm a mom first and I've never wanted to live more than when having children."