Bonjour Brian,
les enfants 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