"Their children are writing."

Translation: Leurs enfants écrivent.

[Aniijay]

I wrote "Leurs enfants sont ecrivent"... no?

[sslawek]

No, the answer is correct. It translates to " Their children write" or "Their children are writing".

[cathyblue]

This confuses me, I used Ses enfants intead of Leurs

[PatrickJaye]

In French the possessive adjective agrees in gender and number with the noun possessed not with the person who possesses.

"Enfant" is masculine so:-

His/her child = "son enfant"

His/her children = "ses enfants"

Their child = "leur enfant"

Their children = "leurs enfants"

Checkout link for full explanation

http://french.about.com/od/grammar/a/adjectives_possessive.htm