Presumably you've had a look here - http://www.hse.gov.uk/youngpeople/risks/ One thing I think you should look at is transportation. If the subbies use a transit van with seating for 3, and the WE person will make the team 4 strong, how will they be transported between sites. Hopefully not in the back of the van. you might also want to check the vehicles are legal in terms of insurance, tax and license. Not strictly your responsibility in the normal course of events but you are assuming the responsibility for the child. From claims encountered (I work in insurance) I'd say the biggest potential danger is lack of maturity and activities like disposal of old fittings. I can imagine a child would take great delight in smashing up old lavatories and cisterns and some of the old cisterns and fittings can contain asbestos. I remember one claim involved a double glazing firm where one WE lad decided it would be great fun to break the glass with kung fu type kicks before placing in the skip. Result was a severed achillees tendon.