I actually think there should be a ruling against it. And no way should a mother/father be allowed to have their own child in their class.
No matter how you look at it, your child is different from the others by definition of your presence. Some will find it an advantage, some will find it a disadvantage, but they will not be treated exactly like the others. They can't be. Even if it's just that your child has been a little swine and the teacher stops you in the corridor to tell you, and you then take Junior home and lock him under the stairs...maybe if Junior's Mum hadn't been in the same school she wouldn't have found out, or certainly not in that way....
He may not be treated better, or worse, but he will be treated differently. Even if it isn't overtly done.
I have just started a new class of 11 yr olds. It's a special project in the afternoons and supposedly, the children who are showing real promise in English get to be entered in a draw to do this thing (it's funded by the EU) My first lesson, great kids, all happy and motivated, except one. Sulky and disruptive all lesson, did badly in the pre-course test we are obliged to give them, and I'm thinking "why on earth is he here?". Because he's the English teacher's son of course.
In the new group I've got tomorrow, there is my partner's nephew. I'm not happy even about that. They asked me if I wanted him excluded from the project this year, but how could I say no? That would be punishing him for his connections. But I really would be happier if he weren't there, and tbh, I bet he would too.