So, in Romanian you don't say things like "E ai lui, nu ai mei" but instead "E ai lui, nu sunt ai mei"?
Nope, this sentence could also be "Copiii sunt ai lui, nu ai mei."
Copiii being plural should be replaced by "sunt" . Sunt ai lui, nu ai mei.
why this wrong??? "the cildren are his, are not mine"
take that one up with English grammar.
you could say "...,they are not mine" or "...,not mine". but you can't use "are" without the "they"