Having child-tickets is extremely useful when it comes to
managing multiple releases (ie. a single 'bug-report' ticket and
a single 'bug-fix' ticket for each milestone/branch of
development), for managing sub-tasks of an issue and for managing
'bug-fixes' required when developing a new (larger) enhancement.

This plugin modifies the ticket description box and adds a child
ticket listing table and a 'create' button for adding new child
tickets. It has the following features:

It is possible to control in trac.ini the following aspects of
child-ticket creation/viewing:

    allow/disallow child-tickets for a certain type of ticket
    to define the table headers displayed in the parent ticket
    to define a default for the child type to be created
    restrict the type of child-tickets
    to define which fields are inherited by child-tickets

WWW: http://trac-hacks.org/wiki/ChildTicketsPlugin