Would you stay for the child?

If you’re referring to not being happy with the relationship, better to be from a broken home than to live in one. Modeling an unhealthy relationship for the child does them no favors. Leaves them thinking it’s the norm, which could be very detrimental especially when they begin to seek out their own life partner.

No, why would I do that ? To suffer until they're 18 and immediately get divorced or something? To lie to my kid about the foundation of their reality? Besides I doubt I'd be able to fake it for that long if I was actually miserable I don't do that kinda thing anymore.

And considering my own parents? I'm glad they separated when I was a kid because while I didn't understand everything I did understand that they weren't happy together and that their relationship was for themselves. It wasn't for me the bastard child lmao 💀

I did and it was a mistake. Two happy homes are much better for the children than one miserable home

I think if a marriage is unhappy and can't learn how to mend the problems, then it's better for the kids if the parents separate.
Reasonably happy adults are much better parents. They don't speak ill of their ex's, accept each other's new partners, cooperate on child care, and work to create a mature and politely friendly relationship.
However, every couple is different. Sometimes one or both parents have serious and potentially dangerous problems. That's usually where the law has to step in and very often things don't work out well. Such people were probably ill-suited to become parents in the first place.

The longer a child is exposed to a toxic home environment, the more long term damage they will suffer.

im grateful my parents got divorced

Think it depends on the situation if you think its a dangerous or bad lifestyle for the child then you should leave, had friends who just knew they were not right for each other but waited until kids were out of school

Look around. The nuclear family is a human centipede of trauma.

The best thing you can do for a child is give them a healthy, loving environment to develop in, whatever that looks like.

If the parents are divorced it doesn't mean one of them stops being a parent.

What if they are both unhappy and stay for the children?

For as long as I could. They’re the centre of my world.

That depends on alot. Normally i would say no. Not if parents are unhappy but to teach your child why and explain , not to let them think whatever. My husband didnt even talk to his kids to sit them down with their mom. I think that could do real damage, emotionally.

If any parent isn't really happy or wanting to he around the kid, I'm pretty sure a legit threat/concern could be them mistreating or hurting/killing said kid, or the other parent.
My siblings and I have one parent and we're fine