If you really feel you need to send something out I would just send out a save-the-date for the at home reception- that way it focues on what they are invited to not on what they are left out of. It doesn't have to be anything fancy just the basics...
Amanda and Eric will be getting married
at Walt Disney World
(you could say small private ceremony but it's not necessary- you could also put your wedding date but then they may try to crash).
Please save the date
to celebrate with them
on (this date)
at (this place- at home reception site).
I wouldn't go into the details of why they aren't invited to the wedding portion of it... just keep it simple and state it in a factual matter. It's your wedding and you don't need to give them a list of excuses as to why you're doing things the way you are. I'm sure if they have questions they will talk to you. Just keep the focus on the at home reception that they will be included in and not on the ceremony that they won't be included in and hopefully it will go over ok! Good luck!