That song is the nursery rhyme "London Bridge is Falling Down," or simply "My Fair Lady."
London ba shi o chi ru, o chi ru, o chi ru
London ba shi o chi ru, My Fair Lady
(London Bridge is falling down / Falling down, Falling down. / London Bridge is falling down / My fair lady.)
Tetsu to hagane de tsukure, tsukure, tsukure.
Tetsu to hagane de tsukure, My Fair Lady
(Build it up with iron and steel, / Iron and steel, iron and steel, / Build it up with iron and steel, / My fair lady.)
Tetsu to hagane ja magaru, magaru, magaru.
Tetsu to hagane ja magaru, My Fair Lady
(Iron and steel will bend and bow, / Bend and bow, bend and bow, / Iron and steel will bend and bow, / My fair lady.)
Kin to gin de tsukure, tsukure, tsukure.
Kin to gin de tsukure, My Fair Lady
(Build it up with silver and gold, / Silver and gold, silver and gold, / Build it up with silver and gold, / My fair lady.)
Kin to gin ja nusumareru, nusumareru, nusumareru.
Kin to gin ja nusumareru, My Fair Lady
(Silver and gold will be stolen away, / Stolen away, stolen away, / Silver and gold will be stolen away, / My fair lady.)
Rou to ishi de tsukure, tsukure, tsukure.
Rou to ishi de tsukure, My Fair Lady.
(Build it up with bricks and mortar, / Bricks and mortar, bricks and mortar, / Build it up with bricks and mortar, / My fair lady.)
Rou to ishi ja kuchihateru, kuchihateru, kuchihateru.
Rou to ishi ja kuchihateru, My Fair Lady.
(Bricks and mortar will not stay, / Will not stay, will not stay, / Bricks and mortar will not stay, / My fair lady.)