Yes, platies eat their babies. Due to the stressful and exhausting nature of the platy fish birthing process, the mother is very hungry when it’s over. Sometimes, the easiest and quickest meal she can get consists of her own fry. Male platies will also eat the fry if given a chance.
To maximize the amount of surviving platy fry, you should move the mother into a breeding box or even a separate tank. Monitor her as she’s giving birth, and when she’s done spawning, separate her from the babies immediately.