Your sentence is very close! Here's a corrected version with proper grammar:
Corrected sentence: Daddy seahorses carry the eggs in their belly pouches until the baby seahorses hatch.
Explanation:
- Use seahorses (plural) with carry because we are talking about more than one daddy seahorse.
- Their matches the plural seahorses, replacing his.
- Belly pouches is plural to match the plural subject.
- Use until instead of till for clearer formal writing, although till is also acceptable in informal contexts.