It is not an idiom nor is it a standard phrase. The numbers can be anything, except the second number cannot be less than the first. It is an abbreviation for "0 successes/wins for 7 attempts/tries" 0 for seven 1 for 5 10 for 20 90 for 100 3 for 3

More generally, the status of whatever we're talking about is [x] successes after [y] attempts. So if we played three games of cards, and you won two of them, you could say you're two for three playing cards against me. If you won all of them (and I lost all of them), I could say I'm zero for three (or "oh" for three) playing against Anna.

It's not specific to American English. Say there are seven questions. And a person gets all seven questions wrong. Then they are 0 for 7. Or a sports team has played seven matches and lost all of them. Same thing.

To answer your second question if you said: "We are 0 for 7 getting kids "with virtue" from the factory" I would wonder "Why are you getting kids from the factory? What is a "kid with virtue"? What kind of weird things do they do in Ukraine?" But I would be very clear that you made 7 attempts do do whatever it is you were doing and none of them had been successful. The first half of your sentence is very clear. The second half is not. What is a kid with virtue? and why do you get them from factories?

It is a common, short way of saying 0 successes in 7 tries. If we succeeded on the next try, it would be "one for eight," then another success would make it "two for nine," and so forth. Another way to say it is "none out of seven, one out of eight, two out of nine." By the way, in US English we usually read "0 for 7" as "oh for seven," not "zero for seven." In US English it is very common to read the digit 0, zero, as if it were the letter O, oh. In fact old-timers may remember a time when typewriters did not have keys for either 0 or 1--when a zero was needed, a capital O was typed; and when a 1 was needed, a lower-case L, l, was typed.