Place apples, rhubarb, water and 1 tsp of sugar in a small saucepan. Cover and cook over a low heat for 4-6 minutes, or until fruit is soft. Remove from heat.
Combine Farex® dry cereal and remaining sugar in a bowl. Rub in butter with fingertips until evenly distributed.
Spoon fruit into a lightly greased oven proof dish. Top with crumble mixture and bake at 180ºC for 8-10 minutes or until golden. Cool slightly before serving with baby custard or yoghurt.