Cabbageleaves, I didn't follow the lad around at all, at first and just commented to his dad, once I'd realised he was there, that it was nice for his son to be learning the trade. I left him to it until he began shouting as water spurted out from a radiator. I'd assumed that he probably knew more than I did about heating systems and had jokingly complimented him on that to make him feel good.
However, it then became clear that he hadn't really done much like this before, didn't know about heating systems (he said this to me) and that his dad and workmate weren't responding to his calls. It was only after this that I felt obliged to be around when he went into each of the rooms.
As time went on and my own DCs needed me and were fretting about the boy standing on their toys etc etc (I'd told mine not to go near the works so they were upset that the other child was allowed out there but they weren't), I felt more concerned and angry that the workman hadn't supervised his son, hadn't 'trained' his son and wasn't responding to his son when he called out.
Once I realised that things were going wrong and that there was no one supervising, then yes, I'd have felt obliged to be around too, even if he were 22. However, I'd have felt more easy about going to the workman and expressing some concerns to him.
As this was his little boy, I felt more obliged to be 'motherly' and chatty with the boy and unable to speak to the father about him, as all parents are protective of their children and I'm sure he'd have taken huge offense if I'd said anything negative.
So I felt really 'trapped' by the situation, with obligation to the boy - given it wasn't his fault he was being told to do stuff he didn't really know how to do, obligation to my own DCs who I had to leave alone and unhappy, obligation to protect my property, need to keep the workman 'happy' as he's still in the middle of the job and I don't want to 'put his back up' a this stage.
I came on here to see if there some straightforward rules and regulations regarding children at work and untrained minors working unsupervised so that I might be able to find an unemotive, calm way of stating my concern to the man, eg "hey, I'm really sorry but I don't think your son should really be here and working unsupervised because there are rules that indicate this isn't safe...and also what about your insurance?"
However, it seems there aren't any rules like this from what people are saying and I'll just have to try to field all the conflicting obligations without upsetting anyone, as best I can. I felt sorry for the little boy as he was very pleasant and was clearly doing his best. It's just that the situation wasn't at all ideal for me.