Showing child taxonomy terms if the parent is selected

Hi,

I want to show one drop-down with a list of parent vocabulary terms and then when one of those is clicked the second drop-down is displayed with its child terms.

You can do this by setting up a filter for each parent taxonomy term and then selecting the children it should show.

I was wondering if there was a more elegant way of doing this so that when the parent is clicked it automatically knows which children to display rather than having to choose each one?