In a small bowl, whisk the olive oil with the lemon juice, salt and pepper until thoroughly mixed.
Toss the arugula with half of the dressing. Divide evenly among 4 salad plates. Toss the mushrooms with the Parmesan and remaining dressing. Taste and adjust for salt and pepper. Spoon mushrooms onto arugula. Serve immediately.
HELPING HANDS: Kids can help mix the mushrooms with the dressing.