Sorry, I used to have a "takeout menu" for summer training in these events (you know, each week to include "one from Column A, three from Column B, two from Column C") but it's not readily available any more.
Assuming that this is a kid who doesn't have a summer competitive season, his/her focus should be on building general athletic ability, through such activities as games, weight lifting, and so on. Avoid a big emphasis on running; especially avoid doing much distance running, which can change the character of a sprinter's fast-twitch muscle fibers--I'd rather see the aerobic conditioning developed through a variety of activities, including swimming, bike, etc. However, some running in the summer is good and one kind of running is very important: retaining the skill of top-speed sprinting.
Because flat-out sprints may not be a good idea during the off-season, I usually recommend accelerations: start at half speed; gradually accelerate over 50m or so while retaining complete relaxation (= control); touch 90-100% of top speed; hold that speed for just a few steps, still with no strain; then ease off gradually. Walk back slowly, until entirely recovered, then repeat. If there's a wind, do these with the wind at your back. Number of reps: probably between four and ten, never pushing for that "just one more" (which would probably be of diminished quality). Do two to four sessions of these per fortnight (three may be best for a lot of kids).
Not only do these help with an athlete's readiness for training for the next year; they let the kid do what s/he, as a sprinter, is best at and enjoys most: running fast!