Mako Adventures (2015) online

Two kids, Kyle and Dahlia, are both normal people with normal lives. But, not for long. One day Kyle sees a mermaid swimming and follows it to Mako Island. It's a full moon, and Kyle ends up in the moon pool. Dahlia finds out Kyle's secret, and asks him to take her to Mako Island. But, a few weeks later a siren follows them to Mako Island, and things start to get a little scary. The siren prepares an army, to take over Mako Island. Dahlia and Kyle have to figure out how to save Mako Island.