I didn't think any of the parents had learning difficulties - and they did recognise that laughing at a toddler using swear words can contribute to them using them as a teenager. I thought it was heart breaking to see them all swear at each other, and the dad of that family was physically abusive to his daughter, too. Yes, she was vile....but they were almost as bad, I thought. The parent counsellor's job was to understand that these parents loved their kids and only wanted what was best for them, and to accept that you can't 'make' someone change, but you can change yourself and this can have the desired effects.
Yes, a lot of it starts in toddlerhood. In our house, we have never sworn in front of our kids, and I have had to develop a list of other milder words like 'dickhead' and 'div' that might be used at school, but which I don't want to hear at home because they are not respectful. I wouldn't call any of my kids 'dickhead' or 'div' so I don't want them calling each other that....and certainly not me!
Swearing can become a form of aggression. It's not acceptable in a family. It's ok in the pub, or at school, or among adult friends. but not in the house. Once you have that rule, then stick to it. But if the parents break it all the time, then how can they tell their kids what to do?