You should never restart a race under those circumstances. The hole shot, falls, and general bad luck is part of the race. If your kid has had the good luck and experience to hang behind while the leaders take each other out, his good luck, their bad luck. I have run and promoted races. The only time I red flagged races was times where serious injuries made competition unreasonable. The worse time I ever had was a bad wreck at the finish where the scorekeepers had no idea who placed where. We ran the whole race again only after asking the riders to vote. Use the last good lap as a final finish or run again.