I think it depends on how many people you're inviting and how much you care if they come.
For my dd's upcoming birthday, we're inviting 4 close friends. Since it's just 4 people, we really care that they come. So I sent a save the date email a month ahead of time, and then will follow with real invites to the kids a couple weeks ahead of time. For a bigger party, though, I wouldn't have bothered with the email.
Sarah, mama to Miriam 9/26/2006 and Isaac 2/12/2010