"They are children of the same parents."
Translation:Ei sunt copii ai acelorași părinți.
You need an article, either the definite enclitic one or the possessive article:
Ei sunt copiii acelorași părinți.
Ei sunt copii ai acelorași părinți.
The English sentence has no definite article for children, so I assumed the second version is preferred here.
Thank you for the explanation, have a lingot! I was struggling with this for 20 minutes :D :D :D