Sure, you dont even need dfhack. The data/init/d_init.txt has this setting:
You can have your dwarves start/arrive without any labor types enabled here by setting this to NO. You can also set it to SKILLS to make the labor list set by skill, or BY_UNIT_TYPE to have it done by overall unit type.
[SET_LABOR_LISTS:SKILLS]
This means if a woodcutter arrives as a migrant, he will automatically have woodcutting enabled. Kids would have the skill enabled as well, but since they never learned a skill, they wont arrive with any. But there are two ways around this:
1. Natural skills. My favourite. You could give gnomes 1 natural skill in engineering, mechanics, toolmaking, whatever you see fit. Children would automatically have this labor assigned when they arrive (just like every other migrant as well) and will do the jobs, like any adult. You still cant toggle their labors though, so they are somewhat independant. I think this is an interesting game feature. Kids that might do some jobs, if you want them to or not.
2. Dwarf Therapist. It already has an inbuild option to assign labors to children. Some people consider this cheating. It certainly needs manual input by the player, but allows more freedom of choice.
3. Why not both?
The DT option is always given, even for dwarves, kobolds and orcs, and giving gnomes a few natural skills will make kids more usefull. If you think its unbalanced that they have this 1 free skill level, lower the skill learn rate of that skill by 5%. It evens it out.
And maybe you find some nice ideas in this thread:
http://www.bay12forums.com/smf/index.php?topic=134856.0 Unrelated to the kids, but related to tech and electricity.