Here you go, from a scan of a 1985 US catalog at Brickset:

The text reads, verbatim:
Dear Parents and Children
LEGO® is a brand name that is very special to all of us in the LEGO Group Companies. We would sincerely appreciate your help in keeping it special by referring to our bricks as "LEGO Bricks or Toys" and not just "LEGOS". By doing so, you will be helping to protect and preserve a brand name that stands for quality the world over.
If at any time we can be of service to you regarding our products, please feel free to write to us.
Susan Williams
The text you quote above is slightly different and may be a variant from a different year, or perhaps it's just slightly misremembered.
It's from one of the "mini-catalogs" that came with the sets, so this was both a catalog and a box insert.
I don't see any such message in the 1978 catalog, but the one from 1979 has this:

which reads:
Dear Parents and Children
The word LEGO® is a special one. It's our very own brand name for the brick supplied by our Company, LEGO Systems, Inc.
Look for the name "LEGO" - we're proud of it. It's on every brick we make!
Susan Williams
Susan Williams
Customer Service
The handwritten "Dear Parents and Children" appears to be completely identical, as does the "Susan Williams" signature. But, the famous admonition does not appear.
However, in 1980...

Dear Parents and Children
The word LEGO® is a brand name, and is very special to all of us in the LEGO Group Companies. We would sincerely like your help in keeping it special. Pleas always referring to our bricks as "LEGO Bricks or Toys" and not "LEGOS". By doing so, you will be helping to protect and preserve a brand of which we are very proud, that stands for quality the world over. Thank you!
Susan Williams
Customer Services
This continues (with slight variations in the text) through 1986, but in 1987, the request is gone:
]
Dear Parents and Children
The LEGO Group takes great pride in DUPLO® and LEGO® brand toys, and if we can be of service to you regarding our products, please write to us.
Susan Williams
Customer Services