Ali, have you read all the replies here?
You say It's illegal activity on the landlords property - of course he would do Something You've had a number of helpful replies on that point. What do you think he's going to do? Perhaps try to think about it from his perspective.
I've already explained that, as a landlord, what I'd do is suggest you speak to her and, if that doesn't work, I can tell her you've complained and remind her that her contract prohibits illegal activity and / or you can go to the police. I would not present your complaint as a fact, as I do not know whether it is true, so I would not say 'you have been smoking weed, stop it, or get out' for example.
I might be concerned about whether anything bigger was going on; dealing, setting up a cannabis factory in the spare room; all-night parties that upset all the neighbours, drug use on a scale that attracted dealers, customers and related criminals (thieves etc) but I'd expect to have recieved a few complaints from different neighbours and probably heard from the police if any of that was happening.
My primary concern is; do they pay the rent on time, keep their house in good order, let me know when there's a real problem but not take up lots of my time with trivial nonsense. If you get a bee in your bonnet about this and expect the landlord to act as your agent, you may risk falling foul of the last point and could make yourself the unpopular one.
Your issue is with smoke and would be the same if it was cigarette smoke. That's what you need to be tackling. It's a consideration issue. It's not really the illegality or the nature of the substance that is bothering you (or nothing you have said has suggested this). If your children were teenagers and knew what the smell was, there could be an issue of this behaviour setting a bad example. With smaller children, it's just about smoking but arguably, if you live somewhere with a very small garden, very close to the neighbours, it goes with the territory, however annoying that is. If a bad reaction to your request for consideration escalated into aggression and anti-social behaviour, that would be a concern but again, somthing to talk to the police about, then inform the landlord.