Definition of "merkid"

Here's hoping they get married and spend their days swimming in the sun, merkids in tow.

2009, "Q&A with Aimee Friedman", in Aimee Friedman, Ocean of Secrets, Scholastic Children's Books (2012), unnumbered page

Humpback whales were huge. All they had to do to swallow a mermaid was open their gigantic mouths wide. She was the smallest merkid in their class.

2012, Debbie Dadey, Whale of a Tale, Aladdin, pages 16–17

Every merkid in the entire gymnasium seemed to be holding their breaths.

2015, Janet Gurtler, Cora's Decision, Stone Arch Books, page 30