What is the ruling on a girl, nearly 11 years old, conversing with her relative, who is 18 or 19 years old, and their exchanging phrases such as "my darling" and "my heart"? And is that considered unlawful (haram) and should they be prevented from it?

A girl at this age is close to puberty, and speaking to her with flirtatious expressions is an incitement to fitnah (temptation/strife) and is forbidden. Her guardian must protect her from causes of corruption, for Allah, the Almighty, says: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allah in what He commands them but do what they are commanded." (Quran 66:6). Children are a trust given to parents, and Allah has commanded them to look after their religious and worldly interests, to educate and discipline them, to prevent them from corruption, and to enjoin them to piety.