children as young as seven can take part in bull riding competitions .
youngest children ride calves before graduating to bullocks then bulls .
between four and six , they hone their skills by ` mutton busting ' on sheep .
bull riding is thought to be one of the most dangerous sports in the world .
an estimated one in every 15 bull rides ends in some sort of injury .
unreported world , tonight at 7.30 pm on channel 4 .