Depending on how many kids you'd want to invite, you might want to check out Sport Rock - rockclimbing place in Alexandria. They also have a location in Sterling.
http://www.sportrock.com
It was my son's 6th birthday and kids had a blast climbing. Price is $25 per kid. You bring your own food and cake and they provide gear. Kids get to climb for about hour and a half. There is one instructor per 3-4 kids.