I agree- that morally speaking, we ought to try to avoid deliberately causing others pain and, there's no denying that suicide does hurt others.
However, what are the chances a living, sentient being would suffer to some extent in this world? I'd say- 100%. Of course, the extent of that suffering will vary person to person. How much assistance they get in mitigating harm will vary also.
It's not something that can be accurately predicted but, what's for certain is that some things are inevitable or at least very likely. We will 100% die some day. We are very likely to experience the death/ loss of others. There are also things that- for the best will in the world, our parents won't be able to entirely shield us from. Bullying, illness- physical and mental, violence, sexual assault. So- is birthing a sentient being into a highly volatile situation- which you can't effectively protect it against- moral?
I would argue that it isn't moral to have been brought here. So- we start out in an unfair environment with unfair expectations placed on us. No matter what, we are expected to survive. Many of us are pressured to thrive too. To not become a burden on our parents. No matter what's going on for us. So, to some extent, I see suicide as a correction of their own moral mistake.
Also, presumably they wanted to give us free will. Otherwise- what is this? Slavery? But, free will is just that. We may make choices they don't agree with. We may even make choices that hurt them. Again though- that's something they ought to have considered when they brought independent life here. That it wouldn't always do what they wanted.
Whilst I'm sure many would- be parents have good intentions, they open a can of worms when they choose to procreate. They are effectively inflicting bad things as well as good on that being's life. Why would they even expect their child to stay here if all it does is suffer? Is that moral? Should any of us expect another to stay here and suffer for our own benefit? Where's the morality there? That's hugely cruel!
Parents dumped us in this mess- knowing full well how it could turn out. And- would- be parents do know. They've experienced life themselves- they know how difficult it can be. They also know the chances they are giving their child- their genes, any heriditary illnesses, their finances, their partners/ themselves/ their environment- how likely the child will encounter violence/ bullying. There's a bunch of stuff they could predict.
They also know enough about life to know how unpredictable it is. They would have heard about people being struck down with random horrible illnesses. People being raped. Any number of dreadful things that can happen. So- claiming naivity isn't a valid excuse.
Maybe they thought we would be able to cope with whatever life throws at us- as they have? But then- have they? Can they? How happy are they really?
If you get a parent saying they couldn't go on if they lose their child then- they are conceding that some things in life would be bad enough to make them want to leave it. So- they realise that's possible. Will they think that action is justified? Presumably, they would also be leaving people behind whom they would hurt? So- a double standard also exists there. How moral are double standards? Not at all- surely. You can cope but, I can't. How dare you want to escape the bear trap I've shoved your head into.