They don't have to take one from a kid. Children do just fine with an adult sized kidney.
We're all going on googled snippets at this point, but here's another one, from
here:
LPCH said:
The leading difficulty with transplanting an adult-sized kidney into an infant or small child is supplying it with adequate blood flow. The child’s smaller heart, blood volume and vessels are insufficient to satisfy the blood flow demand of an adult-sized kidney on their own. Blood pooling in the transplanted kidney clots, and the kidney stops functioning.
Our kidney transplant program surgeons solve this problem by placing the child on intravenous and gastric-tube fluids to increase blood volume for at least 12 months after the transplant. To date, this method has achieved one-year survival of every adult-size kidney transplanted into infants and small children.
Based on that site transplanting adult kidneys into children is possible and in fact highly likely to be successful, if and only if you keep them on IV and gastric fluids for at least 12 months. Otherwise it sounds like it doesn't work.
It's not clear whether the statement of the ethicist in question that a transplant was contra-indicated was right or not, but it seems at least possible that the difficulties in keeping a child with serious mental problems on IV and gastric fluids for 12 months might be a factor.
And life per dollar cost...well, that's an alley I won't be going down, thank you very much.
Quite a few people have picked up the idea from somewhere that this is a moral alley you don't go down. I don't quite know where it started or why they believe it, but I'm pretty sure those people have never thought the matter through.
There is a finite medical budget. Some of it's government money and some of it's insurance company money but in the end there is only so much of it to go around. If you don't know what medical problems you are going to have it's rational to want that money spent in whichever way buys the greatest amount of life and health per dollar. If you do anything else, people die unnecessarily.
Of course when it's you or your kid that has a problem that's cost-inefficient to treat, it's also entirely human to think "Screw those other people, I want that cost-inefficient treatment. Who cares if two other people die so that the one person I care about lives?".
Invent a cornucopia that produces unlimited medical resources and sure, you can give everyone the best treatment possible. Until then we have to decide that some people live and some people die, and the best and fairest way to do it is to buy the most cost-effective treatments until the money runs out, and then let the rest die. It's not a happy thought but reality isn't a happy place, and any other possible distribution of the money means more people die or suffer unnecessarily.