I'll begin with this, when your parents abandoned you, didn't come back post apartheid, and didn't try to reconnect in the decades since, they relinquished any and all rights to be involved in your life.

But if you have considered forgiving them and your main concern is if they can be trusted around your kids, have you considered reconnecting with your siblings? Just to hear from them what it was like growing up with your biological parents. You could learn what kind of people they were outside of doing one of the worst things they could ever do to a child.

It doesn't sound like you resent your siblings for their lot in life, so it could open up a relationship with extended relatives for your kids, even if it turns out your parents were shitty people 24/7 and not just to you. If your siblings ask why you're reconnecting all of a sudden you can just be honest too: to give your kids the opportunity to learn about their extended biological family and you haven't decided if that includes your own biological parents.