For a mermaid-themed birthday party, you can make edible mermaid-pearls as snacks.

These are made with a pop-cake recipe, popcakes are those cakes that are on sticks.
The mermaid-pearls are made of crumbled cake, mixed together with a mixture of butter, cream cheese and some icing sugar. This makes the “dough” soft and easy to shape. Make little round pearls in the size you want them to be. You can put a toothpick in them, that you can use as a handle when the pearls are dipped in candy melts. Put the mermaid-pearls in the refrigerator for a while.

Melt some white candy melts, so they get as liquid as possible. You can stir some cocoa butter powder in it, it makes it easier to make it liquid.
Dip the mermaid-pearls in the candy melts one at the time and let them dry on baking paper or by sticking the toothpick into some polystyrene.

For extra effect you can paint the pearls with a bit of mother of pearl powder on a dry brush. Mine is from Wilton, it’s a really small box, but it lasts long.
Take out the toothpicks (if you haven’t done so already) and place the mermaid pearls on seashell before they are served 🙂
Btw, you can make the mermaid-pearls a day or two in advance, but be sure to store them cool and dry (read & follow the instructions on the candy melts package)