How can you track attendance across multiple class hours?

Use that attendance by member consistency report with "Show attendance across all selected groups."

A typical scenario behind this question would be trying to discover whether the Jordan kids are drifting away due to Sunday soccer games or still attending faithfully when you factor in Saturday services.  That is - are they consistently attending when you factor in different worship services or Sunday School times.

Go to Reports / Groups / Attendance By Member Consistency and check "Show attendance across all groups".

CM3 Across Groups

Under Filters / Groups / Group Involvement select People Currently in a group and then Children's Ministry.

Across Filter

Obviously you can filter by any selection of groups.  If you just wanted the groups from a certain age or grade, you could choose those individually.  The result will be an accurate picture of the kids in children's ministry (or whatever filters you choose) and their attendance in any of the chosen groups in a specific week.