I have one calendar in my site and under this site, there are four and five sub sites. each sub site will have one calendar.
I want to display all the events(which are there in sub sites) in my main site calendar.
This can be done in SP 2010 OOTB. Go to the main calendar and click on the left where is says calendars in view. Here you can add additional calendars even from other sites and choose what color they display as on the main calendar.