It is believed the boy hit by an ambulance in Bath is 14-years-old.
He is in a serious condition after suffering a "significant" head injury.
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
Weather: Heavy rain and strong winds in the north
His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."