You've set pop cap, right?
But I liiiike splattering immigrants

You could cheat, and use Tweak or Companion or similar to turn the children into adults, and then off them the normal way. Not sporting, but convenient.
Eh, not really my style. I prefer to do things "Dwarf-natural"

Though if I can't figure it out some other way, it may come to that. I'm loving this fortress and would hate to lose it to the laggabeast.
But aside from that, I would imagine that it might be easier to get adults to go somewhere else than it is to get kids to go somewhere. That is, make a meeting hall area that's got a collapsing ceiling (or whatever). Wait for kids to be there. Draft any adults who are in there and move 'em out. Collapse. This *should* be more reliable than waiting for the child to wander into the deathtrap, because they *should* be drawn to the meeting hall. But you never know.
Oh, actually: children claim their own bedrooms if they can. So, ensure there are enough bedrooms so that each dwarf gets one, then look for the ones that are claimed by children. You could keep watch out, or simply trap 'em so that entering them is lethal.
I might try these suggestions. I like the one with the bedrooms -- then I can be sure to separate the unloved immigrant orphans from "child of megachampion x who has 7000 goblin kills and doesn't really care about anything any more." Although it'd be bad news if they decided to have a sleepover

Step 1: Put pressure plate in child's bedroom.
Step 2: Connect pressure plate to horrible, horrible death trap.
Step 3:

Step 4: Profit!
Too bad the economy's kicked in, or I could actually assign kids to specially constructed rooms with death traps.