The first sentence is not ok. The other two are both good sentences but have different meanings from one another: It's necessary for children to be punished. - In general, it is necessary for children to be punished. It's necessary that the children be punished. - The particular children we are talking about must be punished.

Hola Vicente, The second two sentences sound better, and the first is a little awkward, but I think your teacher is probably right about it being gramatical. Consider these sentences: Children wearing Halloween costumes is charming. Foreigners receiving welfare benefits is unfair. The young couple walking along the beach was a romantic scene. I think those are grammatically OK and equal in structure to your sentence. I could be wrong, though- just my two bits!

Are those children so nasty? :-) I'm kidding. Often I get different answers from different teachers to the same questions and that's quite annoying for a student.I wish I got always the same answer.