Somebody paid for the braces, the cleats and the field trip forms you forgot to hand in. Most of those bills you never saw.

Money spent on you as a child mostly arrived invisibly. The braces, the cleats, the form you forgot to hand in and somebody covered anyway: each of those was a decision made inside a household budget, often at the cost of something else nobody mentioned. You were not meant to see the arithmetic. Hiding it is the point of doing it well.

You do not need to repay it, and most people who did it would be uncomfortable if you tried. Say you know it happened instead. Naming one specific thing they paid for lands much harder than a general thank you.