Pre-puberty we are discouraged from training athletes anaerobically, so we do short/sharp or long/slowish, avoiding 400 m in particular - yet they turn on their aerobic systems quickly, and once beyond the 7 seconds or so of alactic running they are going aerobic. Once in or post puberty is when they will make lots of lactate, and yes this can be uncomfortable until they learn to cope with it. The UKA Endurance ADM actually leaves out the 400 m distance as a competition focus for the multi sport and multi event stages of development - roughly ages 9-12 and 12-15 respectively; so it includes for the multi event stage 100,200,800, and 1500. I would be interested to know how other coaches approach this.