Lets leave aside that there are allegations that the Syrian army committed war crimes, just as there are allegations the FSA committed war crimes.
The US army committed war crimes in Vietnam, Iraq and Libya - are you saying that the US army is not the legitimate army and the US government is not the legitimate government. Of course you could use the get out clause that the US is not a member of the ICC, then again neither is Syria.
In practice war crimes are the fetish of the ignorant and immature - war crimes will always happen in war. The biggest crime of all is war - aggressive war or aggressive proxy war - all other crimes flow from this initial crime.
You were the one who brought up how you disapproved of little children having their throats slit.
That is a war crime.
You can either care about war crimes or not. Most people do.
Many people around the world have decided that it is not OK for a leader to kill tens of thousands of their own people including innocent men, women, and children. That is why we have International laws against war crimes in the first place.
For the case that you brought up, all of the hundreds of witnesses in that case besides the handful of government supplied witnesses, identified Shabiha and regime forces as being responsible for the killings, and the only major investigation found the government supplied story to be "unlikely" due to inconsistencies and errors in their story.
It is one of many, many cases of regime force being identified as having carried out mass killing of innocent men, women, and children in an attempt to terrorize the Syrian people into submission.
I get that you are on the Iranian regimes side, and you want to cheer on the Assad team despite what happens to the Syrian people because it is better that way for Iran's foreign policy/proxy war goals. However, eventually this conflict will end, and those parties like Russia and Iran who could have stopped this conflict in the first week but chose not to do so in favor of pursuing their own foreign policy goals, will have to ask themselves if it was worth it.
Beyond all of the tens of thousands of people who have needlesly died in this conflict, the real important question that has driven those countries actions will be if it helped or hurt their foreign policy goals.
It would clearly be better for both Iran and Russia if Assad ruled Syria with the same iron grip and control that he did so before the conflict, but that is no longer a reasonable possiblity post the current Syrian civil war crisis. Even if Assad succeded in this conflict, he would require an enormous amount of financial and military aid to reatain control which would not be sustainable or desirable for either Iran, or Russia.
For both countries, the sooner they realize that a negotiated settelment that they are apart of in negotiations is the best way for them to achieve their foreign policy goals, the better it would be for them, and the better it would be for the Syrian people (if that matters to them).
Eventually there will be some sollution to this conflict, and the sooner that people get serious about honest analysis of the situation rather than just cheerleading for dicators, the better it will be for everyone.