What is the law on children and seat belts in a lorry ?

I have an old horsebox lorry which does not have any sealtbelts in!! firstly how do I stand in having a child in the fornt with now belt (hasten to add child has never been outside the yard in box purely as I dont know where I stand with the law as cant find anything!!) I do want them put in anyway how would I go about fitting these would a normal car fitter come out to do this? or do I need this to be done at a lorry place? any good construstive advice would be great (as well as 10 points)