OK, my take on it as someone who is both a parent and a former manager of child protection services, albeit in a different jurisdiction. While I personally, think it inappropriate for a 13 year old to be responsible for a 4 year old for the length of a flight to Dubai, on a professional level there are more important things for social services to be investigating than a 4 year old in the care of siblings in the same vicinity as the parents. Social Services in any jurisdiction are stretched enough as it is.
My take would be that one adult should be in UC and one in Y given the age of the children. I would have said go for it but for the 4 year old. It puts a lot of responsibility on the others and also on the cabin crew in allowing access to you if there is a problem.
With regard to the children travelling as UMs there would be a problem with the 4 year old as VS do not accept UMs under the age of 5 even with siblings as they would all be UMs as
under the age of 16 In other words the 4 year old has to be in the care of one or other parent either in UC or Y