Would you find "Je suis super-héros" as something childish, too?
Equally childish, yes. Another awkward thing to say would be
Il est savant-fou, for instance (un savant fou = "a mad doctor / mad scientist"). I can only think of two occasions where it would be acceptable:
1. a child saying
Je veux être + a job (with the "job" being whatever a child may fancy as a job, including a mad doctor), and
2. a gamer playing an RPG and saying that his character's class / job is "savant-fou" :
Je suis savant-fou, Je suis sorcier, Je suis soldat, etc.
Also, I find results on Google for "Je suis humain", but I find NOTHING for "Je suis être humain"; it seems like you have to add "un" here. I don't understand this, either.
With an adjective, no article is required:
humain can be an adjective, so you can say
Je suis humain (just as you might say
Je suis beau, je suis blond, etc.)
On the other hand, an article is needed if you're using a noun. If you're using the word
humain as a noun, you should say
Je suis un humain (the same thing happens in English:
I'm human [adjective] and
I'm a human [noun] are both equally correct).
"Être human" (human being) is a nominal group, so it requires an article :
Je suis un être humain.
I strongly recommend you to forget about the rule saying that no article is needed when the verb is
être. This is highly misleading. Most of the times, when an article is needed in English, it's needed in French too:
I'm a hero, I'm a man, I'm a friend of John's, etc.
The most common exception is
jobs: the article is optional in French because jobs are pretty much considered as adjectives. So the most common way to translate
I'm a doctor is
Je suis médecin.
Note that the article is optional with jobs, but it's not incorrect to use it. The difference between
Je suis médecin and
Je suis un médecin is the same as the difference between
I'm human and
I'm a human in English, or
I'm American and
I'm an American. In other words, leaving out the article is more common, but adding it is required if you wish to add something else:
I'm an ordinary human ("Je suis un humain ordinaire").
