On the menu, click on "Manage" and click on "Groups".
From the list of groups select the group you wish to edit by tapping the "cog" icon.
The edit form appears and you can change the name of the group.
To remove a child from the group click their name and use the arrow to move them to the available section.
To add a child to the group click on the child's name in the available section and click the arrow to move him/her to the "group's children" section.
Click "Submit" to save your changes.