Does it mean what it seems to mean? That is how you tell.
Let's look at it - "Once a man, twice a child" would mean that you are a child twice, and a man once - or, you are childish, then you grow up, then you grow old and become childish again.
So it's not an idiom because an idiom would not make sense unless you knew exactly what it meant. It must be a proverb.
This isn't an idiom because it means exactly what it says. It is an old saying or proverb. Lightning doesn't seem to strike twice in any one spot, so people say that when they hope that something horrible isn't going to happen again.
This is not a biblical proverb but one of man's - born needing help, adulthood stands on their own, old age needing help again is its premise.
The first time I ever heard it was on an episode of Star Trek (the original series). Scotty said it, I believe. Chekov said it was a Russian proverb.
It usually means that the same chance occurrence cannot happen twice.
It hits you twice, a double blow or set back.
Measure twice, cut once..
Trust once betrayed is not easily restored
night at the museum But it's a very old proverb from way back.
It's an old Chinese proverb found in many books of quotations.
The ghost of Jacob Marley quoted Scrooge twice. The first quote was "Mankind was my business. The common welfare was my business; charity, mercy, forbearance, and benevolence were all my business." The second quote was "You are fettered," said Scrooge, trembling. "Tell me why?"
first cousin twice removed
Twice as much as a child.