Bonjour Brian,
- ' les enfant écrivaient leurs lettres' = 'the children wrote their letters to Santa'
→ the children have several letters each
- 'les enfants écrivaient leur lettre' = 'the children wrote their letter to Santa'
→ the children wrote the same letter / there is only one letter
- 'les enfants écrivaient leur lettre' = 'the children wrote their letters to Santa'
→ the children wrote one letter each / have a different letter each
I hope this is helpful.
Bonne journée
