Allow adding child issues with UI similar to "add 'related" function.
In the issue page one can add related issues by entiring issue number.
However, 'Add button' under 'Child issues' goes to create a new issue rather than allowing to add pre-existing issue. Currently if the issues already exists and if I have to add a one or more issues as a child issue of current issue there is no way - I have to go to the individual child issues, update the issue with parent issue field.
A better way could be to have 'Add' and 'New' should be available under child tab.