Regarding the darkening, I guess there would be a million ways of doing it. I suggest trying this:
Add a Gradient Map layer, plain black-to-white.
Clip it to the kids layer. This will mean they're in black-and-white, but the city is still in colour.
Change the GM layer's blend mode to "Multiply". This will turn them back to colour, but a lot darker.
Paint low-opacity black on the mask of the GM layer, to return some of the original brightness where you want the light to be striking the kids.
Then add a black Solid Color layer, and also clip it to the layers below.
Invert its mask to hide it.
Paint with low-opacity white where you want the kids to be darkened.