If the decision was "do something:" I would probably do something designed to punish and deter. E.g. destroy and/or debilitate the command infrastructure that was utilized to order the attack and would be utilized to order future attacks. Palace and Assad himself are fair game. Destroy and/or debilitate the military's ability to manufacture and use chemical weapons in the future. Missile sites, factories, bases. Swift and with a clear directive that these attacks are for violating international laws of war and committing crimes against humanity. Try and get buy in from other foreign countries and the UN but realize political realities might prevent that. Rebel input is helpful but their opinion is not be all, end all if your directive is the enforcement of international war crimes law, because this would not be an intervention designed to turn the tide of a civil war, even if that is a byproduct.