I see it a bit like playing the lottery. Most people would agree that a large amount of money would change their lives substantially for the better. So- why doesn't everyone sink their life savings into it? Because they don't want to lose- obviously. The odds of them losing everything are high. Obviously- the odds of having a child that ends up having an awful life are perhaps lower. Still- on that premise, what risk would someone deem acceptable to put everything they have into the lottery? 5% chance of losing, 10%, 25%?
The thing is though- having a child isn't accepting that risk for you- you're making that choice for someone else. It's like saying to your neighbour- great news! All that money you asked me to look after, I've put in the lottery. All you need to do is pick the winning numbers! Bringing a child here is starting a sequence of events that could either go wonderfully well, or, incredibly wrong. Most likely, a mixture.
Obviously- certain things will have a profound affect on whether that child is likely to do better or worse. A loving and stable environment or the reverse, the presense or lack of hereditary physical and mental illness in the family, the environment they are born in to, the financial situation.
As to- whether non existence is 'good', we don't exactly know for sure. I don't think many of us remember it. So- from what we can conclude, we don't have senses in that state. No feelings, no perception, no pleasure, no pain. A being who hasn't come about yet isn't presumably able to lament on what it's missing out on. But a person in pain, will very likely wish they had been spared it by not coming into existence in the first place.
The biggest problem is choice though. We're not (as far as I'm aware) given the choice to come here in the first place. And then, we aren't freely at least, given the choice to leave. There are all sorts of emotional blackmail and physical means by which our societies make it extremely difficult to leave on our own terms. So basically- we don't get the option to live freely. Living comes with all sorts of conditions that we didn't agree to as well. That our parents basically signed us up for. We're all slaves to capitalism and the law. We live and comply or, we suffer. Some people of course will manage to live and thrive but, it's by no means a certain outcome.