If a child has already been checked in today as Present, changing a child's default group will only take effect tomorrow.  When you check a child in, they are put into their current default group.  Therefore, changing it on their profile after the check-in occurs will not affect their current check-in.  In order to change their group today, simply move them on the status board.  If a child has been checked in as Absent, Sick, Vacation, or Holiday, before the group change was made, you would have to go into the attendance for today to change their group.