Why wouldn’t they help both of their children if they could?

Parents should treat their kids equally, but they can’t always do that. There are good reasons not to always treat your kids exactly the same. One of your kids might need a lot more support to function on a basic level, so you have to give them more attention than a kid who can function well on their own. The problem is that the «independent» kid can often feel left behind and forgotten by their parents, and that is no good. Sometimes, the kid who is getting all the support doesn’t necessarily «need» that support. He knows he will get it from his parents, and he is willing to deliberately act incompetent to get it. Parents who give their children the opportunity to sink or swim are doing them a much greater favor than those who bail them out at the first sign of friction. 

If you’re going to 67 bands to help to one of your children, you shouldn’t leave the other child struggling. If you decide not to help your children with their debt, that’s all well and good, but you shouldn’t pay off one kid’s debt in full and leave the other with no financial help, unless you want the kid you didn’t help to resent you for the rest of their life.