That's the key to dealing with weddings, bar mitzvah's etc. Make it clear in the invitation what you expect of the people you invite to the event.
The funny thing is, I got flack about it from people ;~D
People know how to dress when it's a traditional wedding in a church. If you take that away, some are completely helpless to make decisions. I said... wear what'd you'd wear to a beach party in summer.... is that so hard? :~D
And I also said... do not dress your child in something they can't get wet and build sand castles in. You will NOT be allowed to keep your child from playing because of what you dressed him/her up in!