I just did a quick Google search for miniature sea shells and found this company...
http://www.eatonsseashells.co.uk/cat3.html
They should have appropriate scale shells for you.
For sea grasses, you mostly want pale green or tan colors. You can find those in a hobby train store or cut some blonde kids hair! Hopefully the kid isn't a fighter.
Finally, sand is sand. It's everywhere and easily accessible.
Good luck.