It's not easy to get spokes in the lengths that you need for kid's bike wheel builds, but there are a few options available:
Straight gauge spokes
2.0mm or 14g spokes are available in a range of lengths including the 232-238mm ranges needed for 24" 3x wheel builds. They are cheap but heavy (around 6g a spoke for a 243mm spoke).
Double butted spokes
CX-Rays, Super CX-Rays and DT Swiss Competition (2.0-1.8-2.0mm) spokes are available as short as 232mm.
Make your own shorter spokes
There's really no point in cutting ordinary 2.0mm/14g straight gauge spokes as they are readily available in the shorter lengths, but if you need something lighter, the cutting a longer spoke and re-rolling the threads makes a lot of sense. There are a few options:
- straight gauge Ti spokes - these are 2.0mm/14g and are almost half the weight of a 2.0mm/14g stainless spoke;
- DT Swiss Competition spokes - these are 2.0mm / 1.8mm / 2.0mm. The problem with shortening any sort of butted spoke is that you end up cutting them down into the butted area where the spoke is not of a diameter that you can properly re-roll the thread. For DT Swiss Competition spokes, you can if you cut them shorter than 230mm you lose the entire threaded end butting, but you can re-roll threads on the 1.8mm section and use the 15g nipples.