I believe the age you can grant a child charging privileges is 10, as was the experience for
@scrappinginontario.
It could be that the cast members (although I am surprised this happened at multiple resorts) were confused what you were asking for.
All the registered guests in a Disney resort room (age 10 and up) can charge purchases to that room, as long as they have set up a PIN with the front desk or online. Those charges go to the resort folio, and will be charged overnight before checkout to the card on file at the resort, unless other arrangements are made.
There is also the option to ask that charges for one or more guests go to a different card at checkout. This is common when adult friends are traveling together, as an example. In that case, which has to be set up at the front desk on arrival (can't be done in advance), the guest who wants their charges separated from the lead guest would have to present a credit card to be added to the resort account.
So it could be they thought you were asking if his charges could go to another card? But even then, that card does NOT have to be added to the MDE account. Charges with a MagicBand have nothing to do with MDE, they go to the resort.
But that's the only explanation I can think of.