It is very difficult to resolve this kind of issue without causing some problems. I will give you the legalities.
If he is doing this as a business, he needs planning permission. It doesn’t matter that he is working elsewhere, it depends whether it is a commercial enterprise. It seems not from what you have said that if it is, then he needs planning permission which is a matter for the Local authority Planning department.
With regard to the noise and general nuisance and the fumes, it may be something which the Environmental Health Department would get involved with and whether they do or whether they don’t, you still have a legal claim in common law private nuisance against the father for doing it, the children for doing it, and father for allowing the children to do it on his property.
If you have legal expenses insurance attached to your house insurances you may be able to get any legal costs covered by insurance which removes any risk involved with litigation.
Often, even if you have a quiet word about this, it can potentially spoil the relationship between you and the neighbour but that is across that you would have to bear and it would depend on how softly you approach this.
Can I clarify anything else for you?
Please rate the service positive. It is an important part of the process by which experts get paid. It does not cost you anything but helps us greatly.