"They are babies."
Translation:Ce sont des bébés.
In French, when you have an expression such as "he/she is", either "il/elle est" or "c'est" is used, depending on how the sentence is structured.
Here is a great tutorial on that: http://french.about.com/library/weekly/aa032500.htm
The singular version of the sentence here would be: "c'est un bébé". You have to use "c'est" because it is followed by a modified noun, un bébé. The plural form of "c'est" is "ce sont", hence -- "ce sont des bébés"..
Hope this helps.
"Ils sont bébés" is an unusual construction, where "bébés" is used as an adjective as you would say "they are little".
If you use "bébés" as a noun, you need to change "ils sont" to "ce sont".
- he/she is a baby = c'est un bébé
- they are babies = ce sont des bébés
The same applies with other nouns that can be used as adjectives, like professions and a few others like "élève (pupil), étudiant (student), témoin (witness), victime (victim), membre (member)...".
You were taught that professions (after verbs être and devenir) do not need an article, because of their duality (nouns or adjectives or nouns acting like adjectives):
- he is an actor = il est acteur
In parallel, you were taught that "il/elle est" + modified noun (= determiner + noun) have to change to "c'est + modified noun" ( in plural: "ce sont")
- he is an actor = c'est un acteur.
The alternative translation you mention shows that not only professions can get a single noun after "être" and "devenir". Some nouns, giving the subject a "status" can also behave the same way (ie "real" noun or adjective):
- he is a good student = il est bon élève OR c'est un bon élève
- he is a victim = il est victime OR c'est une victime (une victime is always feminine)
- he is a baby = il est bébé OR c'est un bébé
- he is a witness = il est témoin OR c'est un témoin
- they are good friends = ils sont bons amis (adj) OR ce sont de bons amis
"Ils/Elles sont des" has to change to "Ce sont des".
As a rule, "il/elle est" change to "c'est" and "ils/elles sont" to "ce sont" when they are followed by a modified noun (determiner + noun).
These determiners can be articles, numbers, demonstrative, possessive or indefinite adjectives.
- he is a baby = c'est un bébé
- she is my daughter girl = c'est ma fille
- they are three of my best friends = ce sont trois de mes meilleur(e)s ami(e)s