Redang,
Im crew and as such I am more aware of the many holes in this rule then you can imagine. It is a difficult one and one that needs to be dealt with on the day. But the fact of the matter is this.....No other passengers need to be made aware of the situation! The crew should deal with the child as such and leave everyone else out of it.
Like a broken record...yes the rule is daft, yes its a very grey area BUT its here and we have to obey it until something changes and as such you can pick holes in my apparent "defense" of the rule as much as you like but those are the facts! lol
But to be honest, I dont think I have ever seen a UM actually booked in any cabin other then WT.