crab mother's eat their children like popcorn.

Prey and Predator roles depend on location in the food chain. Just because we as humans eat plants and meat doesn't mean we're prey. Similarly, with certain other animals like fish. Some may eat meat, yet they may still be hunted down by other animals.

they eat mostly see plants, so id say there prey

Any species can be a prey or predator depending on the situation.

I guess it would depend on the type of crab, but most are omnivores, I think.