We own our home, a terraced 2 bedroom. The house next door recently sold to a landlord and he let it out to a new family - a 40 something yr old woman, her 18 yr old daughter who has a young baby and her 19 yr old son (we overheard a conversation which is why I know their ages).
They moved in and later that day, the mum and the daughter were outside smoking massive joints on their front step. In the next few days, i learned that this was a 3-4x daily occurrence, and the smell was really strong in my front room. The houses are from early 1900's so I'm assuming the sealing isn't great hence the smell coming through so easily. I'm on mat leave right now with a 4 month old so I saw them going out several times a day to smoke. They would go outside for 10-15 mins at a time while I could hear her baby crying inside - absolutely broke my heart. My husband went round after about 2 weeks of it and asked if they could smoke out back instead and they apologised and stopped smoking out front.
Now, our bedroom and the baby's bedroom smell like weed. All day, all night. They are very clearly smoking it indoors. He went round a couple of weeks later and very politely explained that our house smells of it and could they smoke out back. The older mum said they don't smoke it indoors and only smoke it out back "because of the baby" 
even went as far as to say it must be coming from 2 doors down (who is a family with 2 children whom we've known for years!). They must be stoned out of their minds the amount they smoke, my husband has said every time he's seen them/spoken to them, they've been bleary eyed and clearly high.
He's fuming and wants to approach them again. He's very understandably irritated that we're being subjected to it constantly. Not exaggerating with I say I can tell every time they smoke and it's several times a day and night. I can smell it when I'm up feeding the baby in the night as well. I'm really reluctant because they're not going to admit it and they're not going to stop, and I don't want friction with them. We've had neighbours before who we had an issue with and it honestly made me ill with stress. We could contact the landlord but it'll be obvious it was us. And I'm not even sure he would care enough to do anything. We see the son with his friends and tbh they seem like the type who wouldn't think twice to tip a tin of paint over our cars or something similar out of retaliation, literally no fear of consequences. And they're home all day as am I, I don't want my life to be even more stressful than it already is as a ftm raising a baby during a pandemic! I have visions of them trying to make our lives hell if we were to push it.
Would you say something or not bother? It eats me up inside letting them "get away with it" but the possible reaction makes me think it's just not worth it. We're looking at moving but it won't happen for at least 1-2 years.
I've googled about weed smell coming through walls and it says that the smell alone isn't harmful but I still worry about my daughter 😔
Opinions welcome please.