There is no right age for introducing sugar - mostly it is personal preference.  However, there are some good reasons to consider limiting the amount of sugar young children eat.  Sugar is a non-nutrient and as such, is not required in our diets at any age.  It is also easy to become addicted to sugar.  Since life-long food preferences are often developed during the first two years of life, offering children plenty of fruits and vegetables, whole grains, and protein and limiting the junk food is usually recommended.  Your relatives may feel that offering this drink is part of the culture they grew up with.  Try to sensitively suggest that it is okay for special occassions (birthdays, holidays) but you don't want your children having this drink on a weekly basis.  You may also want to investigate whether the drink can be made without the addition of sugar.  Ultimately, as the parent, you can decide what kind of diet is best for your children and if you address the issue with understanding and sensitivity, it will minimize family tension.