M.O.
I know you already got responses, but I wanted to share what we did. For my son's 5th birthday we had it at a museum and invited all the kids from his pre-school class. We actually didn't think everyone would come- all but one came. It helped us get to know other families. It got my son some invites to other parties and it set up connections for playdates. One thing that was really nice is how many parents we had just met stuck around and helped with cleaning up. My son felt really special that day.