Unfortunately, the kids discovering they're mermaids thing has been done before, and this begins to sound like every Changeling Fantasy
story I've ever heard. Unless there's some sort of twist, there's nothing here enticing me to read.
As the above poster mentioned, there needs to be a hook, something along the line of an actual conflict. What are the children's goals and what obstacles are they facing? The sickness and the move could be two sources of conflict. But there needs to be a greater dilemma. Are their lives at stake? Is the family experiencing financial trouble? Is there anything that the main characters can be fighting for? Without some sort of conflict, there is no interest.