Original: Crows would love to eat the harvest if not for the scarecrow.
Better (natural): Crows would love to eat the harvest if it weren't for the scarecrow.
Or simpler: If it weren't for the scarecrow, the crows would eat the harvest.
Short explanation for a 6-year-old:
- Both versions mean the same thing: without the scarecrow, the crows would eat the food from the field.
- We often say "if it weren't for" to talk about something that doesn't happen because of something else. That makes the sentence sound a bit more natural.
- Your sentence was almost correct — the small change above just makes it sound nicer.