Granted at that point, you're tossing 2 dwarves away to do what one industrial machine could do...
This IS dwarf fortress.... the little bearded midgets are expendable.
Many people don't like needlessly sacrificing bearded midgets if they can avoid it.
That said, we're talking about nobles, so a) its no loss and b) they were going to meet an Unfortunate Accident anyway, but now said Accident is actually accidental and the noble serves a purpose, even if it is only through their death.
We call that a Win-Win where I'm from.
On what kind of plump helmets are you on? Some nobles are harmless or even useful (mandating production of steel bars,
They'll never do that. I've never seen a preference for 'metal bars', and in 0.34.11, they cannot mandate materials.
Kids, on the other hand, have nothing to do than socialize, getting high skills and probably replacing your 1 good mayor soon after growing up. Considering the recent growing-up bug, they're useless as haulers and soldiers, too.
No, no and no. You can manually replace your mayors whenever you feel like it, there's no need to endure a voted-in kid with bad preferences. Actual long-sitting mayors can reach masterful to legendary social skills, a freshly-grown kid with 'expert' skills won't beat them in the elections.
Self-grown children fare no worse than imported dwarfs in civilian jobs. And why would you waste them on hauling only? Even if you're strapped for resources, native 'peasants' are easy to turn into useful planters, furnace operators or even clothiers. I haven't payed minute attention, but their hauling performance wasn't notably different from that of immigrated dwarfs, i think it's primarily governed by strength and agility, not body size.
Recent research shows that homegrown microdwarfs are complete rubbish when using blunt weapons. They still appear reasonably useful with edged implements.
But even if you don't care about the happiness of your citizens: children are well-connected - they have families who care for their wellbeing. A warning system which depends on children dying _and_ their deaths being noticed sounds like a great way to trigger a tantrum spiral _whenever something dangerous knocks on the fortress gates_.