This should be doable in the get_children() query, try the following:
Take a look at the codex entry of get_children(). You will see that get_children() uses the same args as get_posts(). If you than look at get_posts() you will see that you have 2 order arguments to work with:
'orderby' => 'date',
'order' => 'DESC',
For all possible orderby and order parameters, you can take a look at WP_Query codex.