Hello I have an Unprepared Casters theory. Spoilers ahead.
So the fish person told Deb that they were kidnapping kids for a wizard. Now, what wizard do we already know would have no moral qualms about kidnapping a baby? Mr Mortyss himself.
Now, you may remember that Mortyss's foolproof plan in arc 1 is to wait for the dragon egg to hatch, after which he will ride it and use it to kill the king. He's killed at the end of arc 1, but he doesn't die die until arc 4, 10 years later. And we know he's the one who caused Wrangler's carriage accident, which happens roughly the same time as arc 7. So he very well could be alive.
But why is he kidnapping kids you ask? Well, Mortyss probably hasn't let go of his lovely dragon riding plan quite yet and perhaps has figured out the egg might have hatched now. He may not know what he looks like, but perhaps he was able to figure out where the child is. And logic would dictate that if you steal every child in a town, eventually you'll find the one that's a dragon.
So long story shot, Mortyss is back and having the fish people kidnapping children so he can continue with his 100% foolproof, absolutely no way this can possibly go wrong plan.