I have no idea who killed those little girls. As others have said, it could have been either side.
I know that NATO made an arse of things when they bombed Serbia - a mixture of misfires and poor intelligence.
The poor intelligence was where someone looked at a map and thought that they were looking at military targets, but were actually seeing the names of buildings of historical interest, i.e. the old Admiralty in Šabac.
Also in Šabac, they aimed for the old barracks but hit some of the surrounding houses. Fortunately, according to the locals, the pilot realised that he was in a residential area and managed to stop the detonation of the bomb that landed in someone's back garden.
The also mistook the municipal rubbish dump for a weapons dump and bombed that. It happened out of work hours, so no one was hurt.
None of this made newspapers in the west, so far as I'm aware.
In Novi Sad, they hit the hospital and an ambulance. I'm not sure what the explanation was for that - mistaken intelligence or a misfire?
In Belgrade, there were civilian casualties including children - but I don't think that that was well publicised in the west, although the bombing of the Chinese Embassy by US forces was reported on at length.
So far as the embassy is concerned, the US apparently admitted that they'd hit the wrong building - the actual target was less than 500 metres south.*
I think that such incidents tend to be written off as 'collateral damage' unless it's your own people that have been hurt. I hope to God that a civilised country wouldn't deliberately be responsible for a London blitz or Hamburg bombing these days.
*ETA There's a western theory that some kind of secret guidance system was housed in the embassy, hence the locals being able to take down the American Stealth bomber. Locals say that it was just luck - if there had been a secret system, they'd have taken down more than that one plane.