I recently worked with a mother who has a 12 year old daughter. She was struggling to eat variety and had her favourite foods that she always wanted.
Working with older children can be challenging as they have years of habits to overcome plus a lot of personality! On the plus side they are also quick to understand concepts. So in this case we have changed the food dynamic and got this very athletic young lady eating less processed food and more fresh food.
Her mother remarked that she has noticed a definite change in her daughter's moods. She is less cranky and irritable and more chilled.
And what parent of a tween does not want that?!
This is of course self-replicating. Calmer kids are more open to eating sensibly.
Have you noticed diet as a trigger for moods? I know for me that if I skip a meal I get very cranky (well more cranky than usual!).