I can't be arsed to read all 9 pages so apologies if it has moved on since the OP.
Just saying that my Mom and Dad did this once when I was a child. In their case they were offered an upgrade for themselves at check-in while us kids (ages ranging from about 12 to about 8) stayed in economy.
We were self-sufficient as we flew transatlantic all the time (family both sides of the pond).
Can't see the harm as from the childs point of view it is exciting to have a bit of independence and feel grown up and the seats in economy are a reasonable size for children so they will be perfectly comfortable in them.
As long as you trust them not to run amock in economy and you pop back regularly to check on them.
I imagine that most of these 9 pages have been posts of horror at the idea though.