"The children have a normal head."
Translation:Τα παιδιά έχουν ένα φυσιολογικό κεφάλι.
With ένα it sounds a bit weird, without it it's fine in Greek. So, τα παιδιά έχουν φυσιολογικό κεφάλι is right, with τα παιδιά έχουν φυσιολογικά κεφάλια being correct too, of course. Sometimes, it is even prefered to use the singular. Πχ Οι μπαλαρίνες έχουν λεπτή μέση (it sound weird to say λεπτές μέσες)= Ballerinas have thin waists. It depends on the body part whether you can use singular and/or plural.
Just to say, I think the English version of this should be changed, because it is simply grammatically wrong. Heads has to be in the plural. The only context where it would be possible is if the head is a separate object - "the children have a pig's head", but that might be a bit creepy.
It does sound a bit off in greek as well (personally, I think it sounds even more off with the article ένα there), but in spoken greek, I've heard Greek people mixing up singular and plural in other cases, when they wish to make a general statement.
-Οι καμηλοπαρδάλεις έχουν μακρύ λαιμό - (The) giraffes have long necks.
-Τα άλογα έχουν χαίτη - (The) horses have manes.
-Τα πουκάμισα έχουν γιακά/γιακάδες - (The) shirts have collars.
(I think it's a bit slangy.) I'll definitely keep an eye on this sentence though. We could come up with a less confusing one.^.^