While it isn't proper ettiquette to write the clear message right on your invite, there are several ways to make your decision known.
First off, guests should hopefully know that who is invited is who is listed on the envelope. Whether you are doing a double envelope or not, the outer envelope should clearly be addressed to who is actually invited
ie. Mr. & Mrs. Robert Smith or The Smith Family
But sometimes, couples don't think that will be enough for their guests to get the message. Some write, "Adult only reception"
Finally, the response card also serves as a clue for guests as to who is invited. Some couples opt to write
Really, these rules don't just apply to kids who aren't invited, but also those unmarried friends that you prefer to come solo rather than with a date who is in a committed relationship.
Finally, the response card also serves as a clue for guests as to who is invited. Some couples opt to write
We have reserved _______ seats for you
And then they write on each RSVP card to how many seats are reserved.
Really, these rules don't just apply to kids who aren't invited, but also those unmarried friends that you prefer to come solo rather than with a date who is in a committed relationship.
P.S Guests over the age of 18 should receive their own invite, whether they live at home or have moved out.