We had the same problem when we lived in town with our RB Angus.
The kids would come through the alley where our back yard was. At first he didn't care a bit but after a while he would start barking at them like you say Nicki is. I watched and would see them poke sticks at him and stuff like that through the fence. We moved out of town shortly after that (the dogs being our main moving factor) so that was our solution.
I'd probably keep him indoors at the time the kids are coming home from school.

This doesn't mean that Nicki will be aggressive to all children. Angus loved children even after his ordeal but I'm sure if it would have continued he wouldn't have liked them very much.

Hope you can get this all worked out. Good luck!