"The children are his, not mine."

Translation: Copiii sunt ai lui, nu sunt ai mei.

[PhilipNikolayev]

I believe "Copiii sunt ai săi, nu ai mei" should be accepted.

[alba512857]

why it cannot be : copiii sunt ai sai nu sunt ai mei?

[DavidPering]

Can anyone explain the grammar here?

[Victorya_I]

Maybe you can say "copiii săi"(his children) but you can't say "copiii ai săi", only "copiii lui"(children are his).

[DarrenLaffar]

You are correct. Confirmed by my Romanian wife!
So you don't say "copiii sunt ai săi, nu ai mei" but must use "lui" in this context. So ""copiii sunt ai lui, nu ai mei" is the correct form.
Although the former is not technically incorrect though - it is merely not said!