Some states allow the filing of private dependency (i.e. child removal) petitions. However, you are going to need evidence. If you're sure there's dope in the house, you can offer to babysit Jr. while mom goes pub-crawling and invite the police in for a look. (You have control of the premises, you can consent to a search, unless your state has some screwy idiosyncracies on the issue.)
The best advice so far: (a) contact a family law lawyer FIRST; one familiar with dependency law and (b) contact the local narcotics section. The family lawyer, unless he's a complete idiot, can tell you what you'll need to make sure the cops don't waste their time. You have basically one shot to get the evidence you need to protect the child. [b]I do not advise you to do this[/b] but here, at least, if you could get dope out of mom's house, and you are a solid citizen, the dope plus your statement of its origin would get her an abrupt visit from the folks we so often denigrate here. Even if you do not involve the police, you will need [i]evidence[/i] that her way of life is dangerous to the child. Evidence is what someone [b]who will testify[/b] saw, touched, tasted, or heard come out of [b]mom's[/b] mouth.
If you can't get with a lawyer for whatever reason, an older beat cop can be a valuable source of advice.