For my two daughters 2nd and 4th birthdays, we had a combined "Little Mermaid" party.  We use the computer to make the invitations asking everyone to join us "Under the Sea." I made a cake from a doll cake mold (which looks like a hill) and inserted a mermaid Barbie into the top.  I decorated it to look like a rock, icing on a mermaids tail.  I put this cake on a sheet cake, decorated as water. Ariel joined the party to do face painting and a magic show. We had a mermaid piñata which all kids loved. It was stuffed with inexpensive candy and sea animal figures from the dollar store. The best attraction of the party however was the party favors.  We gave every child a goldfish in a bowl and a little bag of food.  The fish swam around in a kiddy pool during the party!