Here is an idea.
Make a new table with one record in it. It will be [Record ID#] of 1. Put fields in that record for the start and end date. This will be the date selector table.
Make a field in each detailed record of the event dates (those child recO do) and have the formula calculate to 1.
Then make a relationship back to the date selector table.
Lookup the start and end dates from the date selector table to the child table. And make a formula which calculates to true if the dates on the child record falls within the dates selected.
Make a summary field on the main event echoed of its number of children dates which fall between the start and end dates.
So now you can go to the date selector record and enter your dates and get a report of qualifying events. That report can even be embedded in the date selector so that as soon as you save the start and end dates, then the qualifying events will be right in that record as an embedded report.