To move any kids, from your Enterprise account click on Manage in the top menu and select Groups:
Go to the respective group or subgroup where your kids are and click the More Tools drop-down under the Manage Kids menu on the right side:
Click on Move Kids
Choose the group from where you are moving kids and the group you are assigning them to. Check the Select which kids you want to move box to select specific kids:
Click Submit at the bottom of the page to apply the changes!
Check out this article to learn how to delete a group or subgroup: Can I delete organizations/groups/subgroups?