@Lrck282 There shouldn't be a dark form for a mermaid, even if one of his/her parents is a vampire. Such a child should be either a full mermaid or a full vampire. Mermaids do have a 2nd form in CAS so you can edit both their human form and their mermaid form. How it's done is very similar to a vampire so just to be sure, when you mouse over the portrait at the bottom in CAS and get the little choice thing where you can copy the human form to the occult form, does it say dark form or something else?
It does sound like you have a problem though. So have you tried having the children eat mermadic kelp? They'll need to eat 2 one right after the other if you don't already know that. That should change them back to just a human. If it does work, then have them eat 1 mermadic kelp to become a mermaid again and see if the problem goes away. If it doesn't work, then there is another thing that you can do if you have MCCC. There are options to force a change to a sim so that they can become any of the occults as well as to do the opposite and turn an occult into a human. I'm not sure if either of the things I have mentions will work on children though.
I had a very similar problem with an alien/vampire hybrid way back when not too long after Vampires came out. As it ended up, I finally had to delete the sim since it was causing all kinds of problems in my game. That problem was fixed in a patch later on.
