Use ground flaxseed rather than whole flaxseed. Whole flaxseeds do not always break down fully in a standard blender and can leave a gritty texture.
For a smoother finish, blend the oats, chia seeds, flaxseed and milk first for 20 seconds before adding the remaining ingredients. This helps break down the dry ingredients and gives the smoothie a less grainy texture.
Let the smoothie sit for 5 minutes after blending if you like it thicker. The chia seeds and oats will absorb some of the liquid, making it more spoonable.
Use a ripe banana for the best natural sweetness. If your banana is underripe, the smoothie may taste less sweet and slightly more earthy.
Avoid adding too much maple syrup at the start. Frozen berries vary a lot in sweetness, so it is better to blend first, taste, and then add more if needed.
For a colder smoothie, use frozen banana as well as frozen berries. Slice ripe bananas and freeze them in a single layer before transferring to a bag or container.
For children, you may want to start with less chia and flax if they are not used to a seedier texture. You can gradually increase the amount once they are used to it.