Yes, molly fish can eat their own fry after giving birth. This behavior is not uncommon among many species of fish. In an aquarium setting, the young fry are at risk from not only their parents but also other adult fish present in the tank. It's a natural predatory instinct and also a mechanism to control population in confined spaces. To protect the fry, it's essential to provide plenty of hiding places in the aquarium. Dense aquatic plants, caves, and other structures can offer the fry places to hide and escape from larger fish. If you are keen on raising the fry to adulthood, consider moving them to a separate breeding or nursery tank immediately after birth. This will give them a safe environment to grow without the threat of being eaten.