We're all overlooking one thing here though, Navi's in the picture. Is this possibly because he already wanted a fairy, or a prophetic dream of some sort?
Mido teased him and I am sure he felt like an outcast, he probably dreamed of having his own fairy to and that's why he drew it on there. He wanted be a hero, strong and brave but also not to be an outcast. Though it seems other than Mido all the other kids accepted him. But one bully is more than enough.