I have a report pulling out multiple child records, grouped by parent, where i want just the first child record based on created date/time. In SOQL i would use "order by CreatedDate and limit 1". Is there a similar thing i could do in reports to limit the child records?
Unfortunately, no. The Report Builder does not provide a LIMIT option. This will require doing your report through other means, whether a Visualforce page with SOQL or exporting the data to a different system.
You may be able to find something in the AppExchange.