Kyūjitai form of 鉄砲 (“gun”)

merger, union, confederation / annexation, amalgamation

子供心: childlike mind, judgment as a child

Children's Day, a festival held in Japan on 5 May to celebrate children; part of the Golden Week series of holidays in May

theme song

(music) folk song (of the modern American-style "folk music" genre)

song themed around an entity

love song

denpa song, a Japanese genre of catchy electronic music featuring cute vocals and nonsensical or unusual lyrics.

official song themed around a fictional character