Can a writer help me out with this ?

Start with watching Disney yet again and write something different.

yaaaaa.............

he can

Why can't she bring them with her when she marries the Prince? Why does she have to leave her children behind? 

you actually can't! ❤️ she left her kids, and they're going to be emotionally damaged by it, regardless of how important her reasons were. difficult and impossible choices (like choosing between a war and your kids) are a lose-lose. no matter what you choose, someone gets hurt. But that's actually good! characters just need to be an a**hole sometimes! otherwise, they can't learn & grow & change throughout the story. real life people aren't perfect, and they can't always relate to a perfect character. so play into the concept! 

Learn punctuation. Quickly.

I mean she is undertaking the act in an effort to prevent a calamitous war, so people will likely understand 

As is well established by now the mermaid lays her merbaby eggs and then after putting them somewhere safe to grow she can piss off.

Leave the children with a wonderful grandmother or aunt who will dote on them.  Remember dream dates in the Stepmother—-think on that path. 

I doubt you'll be able to completely avoid any judgement against her, but make her relatable as a person to the reader. Show her motivation, show her doubts (if she has any), how much she misses her children and worries about them, how she tries to keep contact with them etc.

This sounds fintastic. What a mermazing idea. Great fish out of water tale. Shell probably come off as a real beach no matter how you attempt to float it. Sea ya.