Regardless, suppose you did lend that child money and they didn't pay you back. Then suppose as young adults, they come back to you and ask to borrow money again. If they always intended to pay you back and have now matured (and maybe even ultimately did pay you back years late), you'd be far more likely to lend them money again than if you found out somehow that he never intended to pay you back the first time.
There is meaningful difference between "This person committed to something and ultimately didn't live up to that commitment," (or only lived up to it for a certain amount of time, etc.) and "This person lied when they said they were committing to something because ultimately they failed." In one case they changed their mind and perhaps knowingly went back on their word or were unable to live up to their word due to forces outside their control. In the second case abuse of trust was the entire plan. Both of these say different things about the person who violated our trust and whether and with what we can trust them in the future.
The parent comment was making a statement about dishonesty in the sense that when an adult makes a commitment, we hold them to that unless we do not trust them, and those were people we thought we could trust. They have gone back on that and now we think they are deceitful because they betrayed our trust. Whether or not they believed it at the time they said it still makes them scumbugs for having changed their mind at a later date.