Tail Making
Get kids playing creatively by encouraging them to make their own mermaid tails. Provide each child with a large drawstring fabric laundry bag, or large sturdy drawstring tall kitchen trash bag (white if possible). Give them silver foil and blue netting to create "scales" and craft glue to stick different shapes on their tails. Provide strands of green ribbon so they can add "seaweed." When they have finished their tails they can climb inside and draw the strings round their waists, to play mermaids.
Mer Race
Have the mer-children lie on their tummies in their sack tails in a row. Place a nautical themed toy, such as a ship or soft toy fish, at the far end of the room or garden. Call out "dive mermaids" to start the race. The children then have to wiggle on their stomachs toward the prize. The first child to reach and collect the toy wins.
Shell Treasure Hunt
Create a treasure hunt for kids to find shells to make into a mermaid tiara. Hide small shells in different places around the room. Hide some under red objects, some under green objects and so on. Give each mer-child a bag. Call out a color and the children have to find objects of that color in the room and collect the shells you've hidden underneath them. Then repeat with another color. The winner is the child with the most shells.
Mermaid Tiara
Give each child a plastic headband and craft glue. The children should then use the shells they have collected in their shell treasure hunt to decorate their tiara. As long as the shells are small they will adhere with ordinary craft glue. Keep a few shells spare for any children who did not manage to find any in their treasure hunt. Once the glue has dried the children can wear their mermaid tiaras.