"Your children are big."
Translation:Copiii tăi sunt mari.
I'm not sure I understand what you're referring to. "Tău" is the possessive adjective for a single possessed object, so "copilul tău" means "your child". "Copiii tău" or "copii tău" or "copii tăi" are not valid constructs, if you encountered one of them, it was a course error.