[[Ditto?startID=5`]]
[[Ditto? &parents=`5`]]
In Ditto 2 its slightly different, it uses &parents now, e.g.
[[Ditto? &parents=`5`]]
http://ditto.modxcms.com/files/snippet-ditto-php.html#parents
... and be sure to use "back ticks", not the "single quotes" for snippet parameters.
hope this helps.