Kids ( myself back in the day) thought this was just a movie about a kid getting bullied who fantasizes about going to meet Godzilla and son. But actually it's about industrialization and it's effects on people and parenting in Japan. Thus, the deeper theme

As for the monsters. Kids got Gebara confused with Gamera (the latter movies not made by the same studio that made this). It is up in the air if Godzilla is actually supposed to exist anywhere in this film or even in the film was he a movie character the kid just dreamed of.

This film also actually has something in common with TZ ep 'Perchance to a dream'. The kid dreams in sequences. Remember when Edward Hall was dreaming of beautiful Maya and the former asked the psychiatrist if it was possible to actually dream that very way!