Should be reporting total, then girls, and then boys as there look to be interaction effects .
Because the KQ-10 is a composite tests there is the likelihood to be some interaction with the sub-domains. Focusing only on the global KQ-10 scores is hiding the subdomain differences to diet. Diet is not expected to have any influence of parent relations, social relations or peers but your study may find an influence on psychological well-being and even school environment. This is the core of your study: does diet have an influence on psychological wellbeing? Remember the KQ-10 is made up of five subdomains (psychological well-being, autonomy and parent relation, social support and peers, and school environment) The fact is diet is no expected to change 3 of these KQ-3 subdomains and only one or two KQ-10 dimensions. Use all five of the KO-10 dimensions as your outcome measure not the Mean average global KQ-10 score. You may have a more important study if you do that, with a different finding to what you have just using the composite total.