dijit树无法从collapase树获取孩子

My dojo tree is very huge. I need to expand one node at time to increase the performance. My tree have check box. When one check a parent node , I need to know all the child of the parent , also the sub child till the leaf of the tree. I am not able to get the child using below code since the tree is not expanded.My dojo tree is very huge. I need to expand one