In the evening, whisk milk, 4 tbsp chia seeds (6 if you prefer a thick pudding), maple syrup and vanilla extract together in a medium bowl. Whisk again just before you go to bed to prevent the seeds from clumping as the pudding thickens. Cover and refrigerate overnight (or up to 3 days).
To make the mango sauce, puree mangoes with juice or water in blender or food processor until smooth. Use right away or refrigerate in an airtight container for up to 3 days.
In the morning, give the chia pudding another good whisk. Spoon layers of pudding, fruit and mango sauce into serving dishes (mason jars work well to see the layers, or unbreakable acrylic glasses are a safe bet for littles).