I usually do not interfer with other peoples parenting practices, however if what you say is true, then there is no parenting being done at all. Although I live in a large metropolis, I still think we should look out for each other and especially the children. My understanding is that if css is called then they are obligated to investigate whether the allegations are true or not, the same is for the police.
If you called only once then I would say to call again and give specifics, perhaps the person who answered the call "dropped the ball"(it wouldn't be the first time the gov't screwed up). Also if you keep track of her ie times of departure, perhaps she leaves at approximately the same time every fri or sat night? That way you can get the police there sooner.
Other than that, if totally frustrated I would threaten the css, ie. If anything, anything happens to this child i'll be the first one to call the papers and the police. Keep records, if only to protect yourself too.
