John,
Richard's suggestion is similar to how San Diego Junior Golf ran (runs?) its events - parents and volunteers keep score in each foursome, with no other adults allowed on the course.
If it's real tournament play, kids must learn to keep each others' score at 13 or 14 years old. Send an adult along to observe, perhaps.
I would support playing by the true rules of golf from the beginning. While the rules can get complicated, they're based in equity, and it's not a total guessing game to figure it out. Compare them to the rules of football, for example.
Besides - water, OB/lost ball, unplayable lie, and sprinklers/cartpath, along with basic tee and flagstick situations, cover 98% of the rules situations out there.
John, that's basically 5 rules, which is how many you suggest anyway!