I know quite a few people who do bushcraft lessons either for children or for adults but how do you start?
Spread the word of what you are doing through your local community and people you know, word spreads fast and you'll likely find a lot of folk who are interested. If you build a good reputation and provide a decent service, people will start talking about it. In my experience, word of mouth is one of the best ways to grow a business.
If you've got the stomach for it, set up a few, easy to run social media accounts. I don't know what's commonly used now, but things like instagram might be useful. Others will know more about that.
what sort of things would be good to demonstrate or teach keeping it sample and profitable?
I would say this totally depends on what your abilities and skillsets are. What's your experience with this kind of thing? Have you got a particular area of expertise?
I could well be wrong, but it seems like the general bushcraft courses are pretty well covered. I'd be tempted to set up something a little more specific. But again, it depends on your experience and skills.
One thought, a lot of people like to come away from these sorts of courses with something they have made. If you can teach people how to, for example, carve and fit an axe handle, knap flint, make cordage or primative tools etc, then those object will hold a special significance for your clients and they will leave with a sense of achievement and pride in their work.