Upvote:2
I can answer one aspect of your question, Or did she just invent it? with a quote from the book version you linked (thank you for the archive.org link). From page 28 (all emphasis mine):
On March 4 (Wednesday) the barricades were re- moved, the "kids" [a popular term for politicians] had come to some agreement. Great?!
This implies that the phrase was something she heard on a regular basis, and she did not 'invent' it.
She does expand on the theme quite effectively in several other locations within the diary:
p. 103
The "kids" really are playing, which is why us kids are not playing, we are living in fear, we are suffer- ing, we are not enjoying the sun and flowers, we are not enjoying our childhood. WE ARE CRYING.
...this winter, the shortage of wood and food, brings me back to earth and then I realize that the war is still on. Why? Why on earth don't those "kids'' come to some agreement? They really are playing games. And it's us they're playing with.
The convoy has fallen through. I think we'll have to give up on it. You can't get out of Sarajevo. Thy won't let you. Who won't? It doesn't even matter . . . We'll stay where we are. This can't go on forever. Perhaps the "kids" will get tired of their game. Your Zlata
Today some Italian journalists asked me what I thought about the idea of "Sarajevo β an Open City." I gave them some answer, but I think the "kids" are just playing and I don't believe them at all and I've had enough of everything. Because, I know there is no electricity, no water, no food, that people keep getting killed...
and maybe the most effective comparison on pg. 179:
The political situation? A STUPID MESS. Maybe that's why everybody is so edgy. The "kids" are trying to come to some agreement again. They're drawing maps, coloring with their crayons, but I think they're crossing out human beings, childhood and everything that's nice and normal. They really are just like kids.
So the original entry on page 28 tells us that this was a phrase she heard used locally on a regular basis, and the author quite effectively carries out the comparison throughout the work.
(I have not been able to get any other searches into the term to get past the mass of general material on the war and the crimes and atrocities committed.)