I think molly fries like a little salt in the water!!! not sure though......senior members will know better than me.
Is it that your Molly fry are being eaten?.. or are they simply dying for no apparent reason?.. If your answer is to the second question, it could be for any number of reasons.. it could be because of an imperfect ratio of females to males (three females to one male is the optimum ratio with livebearers) which could lead to a mother being overly-stressed by her male tank mates and birthing fry that are not at their best healthwise.. inbreeding can also be a problem if the female is breeding with one of her mature offspring, so adding new fish to add new blood is recommended ever so often.. and I'd always suggest, if you're serious about raising the healthiest possible fry, you get a tank that's solely for them.. raise them with regular feedings(three small feedings per day) of the right combinations of food, use a sponge-filter for keeping the water fairly clean, and do partial water changes at least every two weeks.