a "contact high" isn't what you think it is. a contact high does not come from inhaling second hand smoke, it comes from being around other people who are high (could be from anything, including jumping out of a plane). one can almost get the same buzz from just being around people who are "feeling" a certain way. it's an emotional state. you don't actually get high from a contact high, you just feel that way.
you are right, it is rude to be toking around kids. you should be a man, and walk right up, and respectfully remind them that you have kids with you and you would appreciate it if they were more discrete. you would be surprised at how effective talking to someone with respect can be- mind you, using anger and intimidation usually doesn't work (unless you are 6-8 and 300 lbs). it's much better than taking that anger with you all the way home, and writing a rant on a message board about it. carrying that anger with you for so long can only lead to grey hairs and sometimes tumors. in the end, you'll feel better that you at least tried to do something about it.
the difference between marijuana and alcohol is that one is taxed and the other isn't. drunk people are more annoying as well.
the reality of the situation is that any public event with 50K++ people around, you are going to get all kinds- drunks, potheads, bad language, angry-fistfighting types, and just plain rude people. unless you go to a kid specific event, your kids are going to be exposed to things which you may find objectionable. that's just how things are. what's worse, is that at school, they get all that...and more!!!
to all the people who suggested that they get up and get the police- that a bit sissy, isn't it? why wouldn't you want to say something on your own first? what's the worst thing that could happen to you if you said something (keeping in mind that there are 50K people around) ?