If they start fighting a war now and a lot of mermaids are killed, then it will take almost a hundred years for them to bring their population back up to the point it was in before!
And worst of all is the sort of anger the demon lord might develop for them if he finds out that they couldn't handle things properly and they went into a war for something this stupid. It might not even end with a simple warning this time, the demon lord could just eradicate them all from anger! Talia has heard stories about what the former demon lord did to the dragons and some other races that went against him! She didn't want the mermaids to join the list of extinct demons! The demon lord won't even care that his child is now one of them, he would show no mercy to anyone! In the demon world, everyone was guilty if the majority commits a crime, so as far as the demon lord is concerned, every child is also guilty.