The original sentence you wrote is: "Frilled lizards hiss and fan out their frill to scare off a predator."
It is grammatically correct! The sentence clearly tells what the frilled lizards do to protect themselves. It says they 'hiss' and 'fan out their frill' to try to scare away predators.
If you want, you can make it even simpler for a 5-year-old by saying: "Frilled lizards make a hissing sound and spread out their big, colorful frill to scare animals that want to eat them." This makes the idea easier to understand with simple words.