There is no singular for Eltern. Actually, there is the word "Elternteil, but this is very formal and hardly ever crops up. And then we have the word "der/die Erziehungsberechtigte", which, again, is more of a formal/legal term meaning something like "legal guardian". This can also refer to a parent.
In reality, we just say either "Mutter" or "Vater" when referring to one parent. Ditto Großmutter/Großvater.