I think tobacco, alcohol, and marijuana all should be legal for adult consumption but restricted at approximately the level that alcohol is in Pennsylvania (one of the strictest U.S. states):
*Large quantities can be purchased only from a state-run store.
*Small quantities can be purchased from a larger but limited range of businesses and are sold from behind the counter or stored in locked cases. (For instance, you can get a 4-pack of wine coolers from a pizzeria. You should be able to buy tobacco and marijuana in a drugstore.)
*You're not allowed to use it in public, except at licensed establishments. (Marijuana should be sold in "coffeehouses" as in Amsterdam. I'd support prohibiting sales of mj and alcohol in the same establishment, to discourage people from getting too f*cked up.)
*There's an age limit, strictly enforced by those who sell.
*Being troublesomely intoxicated in a public place or DUI can get you arrested. (Neither of these applies to tobacco.)
*Purity and potency are monitored by the state; all additives are listed on the label; potency is labeled using a standardized system.
Also, Tigerchild is right on about switching the drinking age and driving age! The objection people always give me is that then parents would have to drive their teens to all their activities--so? It's an opportunity for conversation and keeping track of where the teen is!!
For tobacco, I think 18 is the right age limit because of the serious consequences of smoking for younger people--I know, it's not good for anyone, but it's worse for kids. I think I'd set it at 18 for mj too.
I don't support banning tobacco entirely, despite its harmfulness and yuckiness, because all that would do is make tobacco addicts into criminals. There are a lot of them, and I'm afraid of what would happen.
For currently-illegal drugs other than marijuana, I think the right approach is to decriminalize possession and adequately fund rehab (so that everyone who WANTS it can get it right away, which currently is not the case in the U.S.) but not make drugs widely available. Maybe you could get them by prescription, which would take care of most of the safety issues.
As for parenting, I don't think being mildly under the influence of a substance in the presence of kids is a problem, but it's best to have one totally sober adult around at all times just in case something goes wrong, esp if kids are little. My mom used to drink every evening while fixing dinner (it helped her back pain) but not to excess, and that was no problem. I do think it's unhealthy to expose kids to second-hand smoke of any kind.