A pool was set up on stage nine during the production (stage nine is one of the largest on Toho's lot in Setagaya). They used the pool, which was about five feet deep, to film the shot of Godzilla rising above the surface of the ocean. Haruo Nakajima (the actor in Godzilla suit) was on a cart on a rail. He held onto the handle of the cart while it was towed by a truck. As the cart moved forward, he slowly rose out of the water. He wore a very tiny air cylinder that contained enough air for about ten minutes. The water that rushed by his face while the cart was being towed made it very difficult for him to prevent the mouthpiece from coming out of his mouth.
Godzilla's look transformed from a part menacing-like look into a loving father after Godzilla vs. the Sea Monster. By this time, it became obvious that Godzilla movies were now being made to suit children. In Godzilla's new look in Son of Godzilla, his eyes were made larger and his snout shorter. The shape of his head resembles that of a human head, probably, to make Godzilla less scary. The tone for Son of Godzilla was a playful and goofy tone, and Godzilla's look contrasted with it beautifully. This suit was later used in Godzilla vs. Gigan in the swimming scenes.
The "Daisenso-Goji" Godzilla suit (first used in Invasion of Astro-Monster (1965)) is used in this movie as a stunt/water suit. It first appears when we first see Godzilla at sea in the film's pre-credits prologue, and again when Godzilla rises out of the water responding to the newborn Minilla's cries.
The name Minira started out as an inside joke short for "Mini-Gojira", because the film crew saw him as a "miniature Gojira". For the English dub, the name was translated to Minilla, meaning "Mini-Godzilla".
Upon its released on home video in Italy during the 70s, the movie was presented as a sequel to the British monster movie Gorgo (1961), with even the title being changed to "Il ritorno di Gorgo" (The Return of Gorgo). The Japanese actors' names were inexplicably changed to "John Wembley", "Keny Sahara", "Dick Kennedy" and "Charles Simon", and the directorial credit was given to Ishirô Honda instead of Jun Fukuda.
Unusually, in the course of his tenure as Godzilla, Haruo Nakajima only played the monster in this film for the water scenes (using the "Daisenso-Goji"/"Nankai-Goji" suit), as a taller actor was needed for the majority of the film to make Godzilla seem even bigger than his son Minilla (thus the new "Musuko-Goji" suit). Tall actor/baseball player Seiji Onaka played the role of Godzilla, but after doing a quarter of his scenes, he had an accident while playing baseball during filming, breaking two fingers. After Onaka bowed out, Nakajima's protege Yû Sekita carried the rest of the role (besides being a good fit for the Godzilla suit).
The film's hero, reporter/photographer Goro Maki (played by Akira Kubo), would be a namesake for two other characters in the Godzilla film series: One is another reporter/photographer hero (played by Ken Tanaka) in Godzilla 1985 (1984), and the other, in Shin Godzilla (2016), as an offscreen scientist seen only in a photo, that of late film director Kihachi Okamoto, in a posthumous but pivotal "role."
Screenwriter Shin'ichi Sekizawa complained that he had run out of ideas for further monster movies, and director Jun Fukuda heartily agreed. Producer Tomoyuki Tanaka then proposed the idea of introducing a son to Godzilla.