"This child is my relative."
Translation:Αυτό το παιδί είναι συγγενής μου.
If an article was used, then it would be implied that there were no other relatives. Contrast with "Αυτός ο άνδρας είναι ο οδηγός μου" - This man is my driver : It is implied that there are no other drivers. To summarise: If it is implied there is only one (e.g. my father) an article is used, otherwise, none.
The concept is a bit similar to "My friend came" Vs "A friend of mine came".
Correct me if I'm wrong, but I think there is no difference between the two in English, so the article 'το', though not necessary, should still be accepted in the exercises where we're meant to translate the sentence to Greek.
My dictionary says that συγγενής can be both noun and adjective. But does it mean that they are declined the same? If yes shouldn't it be συγγενές in this sentence then? Because παιδί is neuter.
You are right about "συγγενής" being both a noun and an adjective but with different meanings. As a noun it refers to a member of your family. In this case it can only be masculine (ο συγγενής in singular / οι συγγενείς in plural) or feminine (η συγγενής in singular / οι συγγενείς in plural). As an adjective (ο συγγενής, η συγγενής, το συγγενές in singular / οι συγγενείς, οι συγγενείς, τα συγγενή in plural) it means something that has common origins or common traits with something else. E.g. συγγενείς γλώσσες (languages that share common origins or common traits) "Συγγενής" as an adjective in medical context can also refer to a congenital disease.