From what I can tell, Projects is the parent and Tasks is the child? If that is right then you wil not be able to look at a specific task completion date as a column on the projects report. One option is to create a formula field that equals "ToText([Completion Date])" and then create a combined text summary field on the projects table that sums this field up from the tasks table. You can then display all child task completion dates as a combined text field.
Otherwise, You can use various summary field options to capture just one completion date from the tasks records. For example, You could get the last date by doing a "max." summary. Or the first date by doing a "min." summary. A note to make is that last I knew date fields were not summarizable, the workaround is the convert the date field to a date/time field, either directly or through a formula rich text field.
Hope this helps.
