Gosh @smellysaurusmum you're having a tough time here.
It's been about 10 years since my days doing a similar role and like yourself many of the staff were flung in the deep end without training.
I know of a few families in my dcs schools who have support staff go in and they're cannabis smokers so your post isn't particularly identifying. I recently delivered some of my dcs clothes to a mum locally also and the smell of weed was pretty strong.
It's sadly really common, people in shitty situations self medicating with weed and then their situations become far worse, horrid cycle.
You report this direct to your line manager, you'd be unlikely expected to personally contact a social worker.
Now I know several social workers both professionally and personally and due to the lack of resources they'd not be looking to do much with the drug use singularly, but combined with a house that's looking a bit chaotic, that might make them react more.
I think we vastly underestimate how many people actually do use weed daily. We're always told it isn't an addictive substance but when you meet habitual smokers who would rather they had enough weed to see them through the weekend as opposed to knowing their kids have enough food in the fridge, I'm personally inclined to disagree.
Agencies are always desperate for staff so I'd be looking for a job with a place that offers a decent training course prior to sending you out on the job.
Or even a company/charity type place that has their own bank staff and offer their organisations in house training.
Not knowing what to do, especially working with children and families leaves not just them but you quite vulnerable.
There are also websites you can do free online training on that you could add to your CV, things such as adversity in childhood etc, so you can market yourself with a bit more understanding of struggling families, might make you feel a bit more confident when applying for other roles elsewhere.
But for your own sake, please don't stay with a company that's not trained you in safeguarding prior to sending you out to vulnerable people.
Good luck and just as a side note, always think, rather safe than sorry when it comes to reporting. Sure, it doesn't make you popular always but ultimately safeguarding is the Job of every responsible adult, not just professionals and I'd something is concerning you/worrying you, tell your manager and do it via email so you can always prove you've done your bit.
Again, best of luck x