"Τα παιδιά τους είναι αγόρια."

Translation: Their children are boys.

[EliasMossholm]

So "είναι" is the same for third person singular and plural?

[G.Georgopoulos]

That's right ;)

[RandyKelso]

Can't see any difference between what I typed (except for accent marks, etc.) and the correct answer, but it was marked wrong. I typed "tα παιδιa τους εiναι αγoρια. Other answers to other challenges without the marks are marked correct, but with typos noted. Are the marks sometimes required to obtain a correct answer?

[mizinamo]

I typed "tα παιδιa τους εiναι αγoρια.

You're mixing alphabets.

The first letter t, the last letter of παιδιά, the first i of είναι, and the o of αγόρια, are Latin/Roman/"English" letters, while the rest is Greek letters.

Use only Greek letters when writing Greek.

[RandyKelso]

You are correct as far as what you see in my comment above, but that's my mistake here. In the lesson it was typed correctly and the word "τουσ"was flagged. I now know that it has to do with my error of using the "σ"character rather than the terminal "ς". Thank you for your help.

[Rodrigo188394]

Why not ' his children?

[mizinamo]

Why not ' his children?

Because that would have been τα παιδιά του and not τα παιδιά τους.

[Vemund75118]

Τα παιδια δικα τους είναι αγορια. Is that also right?

[Sterg19]

Όχι. No, because this form is the strong form if the possessive pronoun. I mean that it means "their own" in this case. It should be τα δικά τους παιδιά, their own children.