Corrected sentences
Option 1 (talking about many crocodiles): Crocodiles have mouths full of teeth! You should keep your feet out of the water.
Option 2 (talking about one crocodile): A crocodile has a mouth full of teeth! You should keep your feet out of the water.
Why these changes?
- Plural agreement: The original sentence said "Crocodiles have a mouth full of teeth." When you talk about more than one crocodile, use the plural noun "mouths". So say "Crocodiles have mouths..." If you mean just one crocodile, start with "A crocodile has..."
- Complete sentence: "Better keep your feet out of the water" is okay for talking, but it is clearer to add who should do it. So say "You should keep your feet out of the water" or "You'd better keep your feet out of the water."
- Punctuation: The exclamation mark is fine because the idea is strong or exciting. Keep it if you want to show warning or surprise.
Quick tip
If you want something shorter and very clear for a child, you can say: "Don't put your feet in the water. Crocodiles have sharp teeth!"