Heat the oven: Preheat to 375°F (190°C). Lightly grease an 8-inch square baking dish or a similar 2-quart dish. This prevents sticking and makes serving easier.

Mix the filling: In a big bowl, combine rhubarb, sugar, cornstarch, vanilla, lemon juice, and a pinch of salt. Stir until the rhubarb is evenly coated. Kids can help with the stirring—just remind them to keep it gentle.

Make the crumble topping: In a separate bowl, add oats, flour, brown sugar, cinnamon, and salt. Stir to combine. Add the cold butter cubes. Using clean hands or a pastry cutter, rub the butter into the dry ingredients until the mixture looks like coarse crumbs with a few pea-sized bits. This is a perfect hands-on job for kids.

Assemble: Pour the rhubarb filling into the prepared baking dish. Sprinkle the crumble topping evenly over the fruit, covering it completely. Let kids “snowfall” the topping for even coverage.

Bake: Place the dish on a baking sheet to catch any drips. Bake for 35–45 minutes, until the topping is golden and the filling is bubbling at the edges. If using frozen rhubarb, it may need a few extra minutes.

Rest before serving: Let the crumble cool for at least 15 minutes. The filling will thicken as it rests, making it easier to scoop.

Serve: Enjoy warm with ice cream, whipped cream, or yogurt. It’s also great at room temperature.