It should be viewed as a privilege. This is NOT an inalienable, granted from on high, God given right. If the Instructor or Divemaster were NOT comfortable with the child in the water, I would respect and defend their position.
There are children (as there are adults) out there that need to be discouraged from getting NEAR water. There also are children that are naturals in water and would be missing out on the rest of the planet without SCUBA. It is the latter group for whom I voted should.
While Im unfamiliar with the Rangers, I think the idea of exposing children at that age to SCUBA is a great adventure. I also commend you, mudguppy for considering giving your time (I assume your not getting paid much anyway) to the kids.
While I dont play a parent on TV, I am one in real life. I have a 9-year-old that loves to dive and is very comfortable in the water with SCUBA. One of the benefits that Ive noticed is that my childs confidence and ability in her schoolwork have markedly improved. After reading Dads PADI OW book (and being conversant in some of the terminology) and staying alive after being submerged for half an hour at a time, Reading, Riting and Rithmatic seem to come easier.
I think every parent considering SCUBA MUST look at each child and decide for themselves whether the child is mature enough physically, mentally and emotionally before they are allowed to dive.