Hmm. A couple things about marihuana:
1. Unless you've grown & cultivated it yourself, you can't even say it is pure marihuana. Most stuff sold on the street is NOT, only medicinal is. A lot of marihuana is laced with a variety of substances, it is very common. You may think you know your dealer, but that stuff is usually passed through about 3-4 people before you get it. At least.
2. Breathing in smoke - any smoke - is not good for the lungs. This is a rather obvious point. If you or your OH like to smoke it, whatever, you are an adult and you are making the choice. I'm not sure why people would not be bothered that a mother would be upset that there is a lingering odor of marihuana in their child's bedroom. People have the right to not have illegal and harmful (smoke is harmful) substances in their own child's bedroom.
I didn't sift through all the posts, but I see the OP was going to speak to them. Good idea, I think. I'm sure they will be reasonable as a lot of marihuana users don't seem to realize how strong the odor can be and how much it can travel.
This is actually a common complaint at my job (I work in a police department too), and yes while it is illegal (although I could personally care less what people do on their own time), I think this mother has a right to not have her child exposed to a harmful illegal substance in her own home. Smoke pot, whatever, but do have consideration for your neighbours.
I'm not sure why someone would think that inhaling a substance into your lungs is not harmful. If you didn't grow it and cultivate it, you don't know what's in it. Even if it is pure for medicinal purposes, smoke is still harmful - medicinal purposes are for people whose illness (cancer, etc) is more severe than the risk from inhaling smoke. It's still harmful to them too - but it's a cost/benefit issue. If you choose to do it recreationally, that's your choice to make (after all, cigarettes and alcohol are not good for you either but all things in moderation I suppose), don't try and sugarcoat that it has no harmful properties. It does. You just decide what is reasonable for you or what isn't.