1.First, have your child make a marinade for the meat by measuring the soy sauce, rice wine vinegar, sugar, and sesame oil into a medium-size mixing bowl. Add the cornstarch and blend it in with the back of a spoon. Stir in the garlic and ginger.
2.Trim any fat or gristle from the steak. Then demonstrate how to cut the meat into pieces about 1 1/2 inches long and not quite 1/4 inch thick. This is a good time to explain that ingredients for a stir-fry are cut small so they will cook quickly (and be chopstick friendly). Stir the pieces into the marinade. Cover the bowl with plastic wrap and set it aside at room temperature for 30 minutes.
3.While the meat marinates, cut the broccoli into small florets. If you like, peel and dice some of the stems and use them in the dish too. Set the broccoli aside.
4.Heat a wok or large saute pan until it's hot. If you're using a wok, drizzle the 2 tablespoons of oil around the insides near the top (it will run down and coat the sides sufficiently). Otherwise, gently rotate the pan to evenly spread the oil. Add the meat, spreading it out in the pan, and stir fry it for 2 to 3 minutes or until it is well browned. Use a slotted spoon to transfer the cooked meat to a plate.
5.Immediately add the broccoli and the beef stock to the pan. Cover the pan and cook the mixture over high heat for 2 minutes, stirring occasionally. Uncover the pan and stir in the meat and oyster sauce. Cook the stir fry uncovered for just another minute or two, until the broccoli is just tender. makes 4 servings.