To change classes for more than one child you would go to the Family tab > Family List > Select the children you wish to change classes (filter if need be) > Click the top drop down (3 dots) > Select Change Classes. Select the class they are withdrawing from (or all enrollments), the withdrawal (end) date, withdrawal reason, new class, schedule, tuition and start date. Click Finish when you are done. 

Note: this will only work if they all have the same new class, schedule, tuition and start date.