Definitely NOT a good idea, its one thing if want to carry your own children in the back thats your decision, or even carry sensible adults...but to carry Other peoples unrestrained Children! on unsafe side facing bench seats in the back of an unsuitable open RL cargo truck on the road! NOT a good idea!
Check with your insurance company, I doubt if they would extend insurance cover for any passengers riding in the truck open back in the event of an accident, I suppose if you got all the other Parents to sign disclaimers... I don't think so.
Due to the RL's weight, which is over 3.5tons gvw, its a heavy commercial goods vehicle regardless even if historic, you are restricted to a Driver and a Maximum of 8 passengers Only, to carry more you need a PSV (Bus/coach) licence, yes even for private non commercial use, it may seem a lovely innocent idea, ...what can possibly go wrong..lots