It has already been mentioned, but I'll second the nomination - the Golden
Guide to Seashells of the World can't be beat for youngsters.  It is one of
the "Golden Guide Series".  This little book has a short introductory
section, with good general information, written in simple language, while
the rest of the book has a minimum of text, and clear, sharp full color
pictures of over 500 species (not photographs, but excellent, realistic full
color drawings).  It used to cost $1.50 (ha - so did a lot of other things!)
but still costs only a few dollars.  It was my first shell book, and no
doubt the first shell book of many other collectors.
As far as starter collections go, I listed several different small
"ready-made" collections some years ago, but after listing them for a couple
of years I got little response, so I dropped them.  I think the problem was
that people who were viewing my lists were already collectors, and into
acquiring shells on a "per specimen" basis, while parents, teachers, and
others who might be very interested in obtaining such a starter collection
for a child were not viewing my lists.  I think such a small starter
collection might be popular if offered through a more general outlet, rather
than a shell dealer's list - something like a museum gift shop perhaps, or
an ad in a general natural history magazine.
Regards,
Paul M.