1) You might get WDW to use the parent's card for the kid.
2) Bringing a note from the other parents should help.
3) When you check-in, ask for SHARE, as below:
. . . Every room key with charging can be tied to a different credit card.
. . . Those arriving at same time or even later can tie their key to their own credit card.
. . . Have them ask for a "SHARE" when they check-in. (Use this specific word.)
. . . If CM doesn't know how, ask them to check the Front Desk Manual.
Example:
Mom and daughter in same room.
Mom can have her room key tied to the mother's MasterCard for mother's purchases.
Daughter can have her room key tied to the daughter's VISA for daughter's purchases.
Process:
. . . mom checks in, with her credit card, just as usual and gets room #1212
. . . daughter tells Front Desk CM this is a "SHARE" room
. . . daughter must use this exact phrase, as it has special meaning
. . . daughter checks-in using her credit card and gets a virtual room #1212a
. . . repeat for as many credit cards room people want to use
. . . mother and daughter (and others) purchases now go to individual credit cards
. . . one or both could pay a part of the room bill
. . . the 1st to check-in gets charges for phones or room service (system glitch)
. . . this can be done for as many room keys that have charge privileges
NOTE: The alternative is to segregate his charges by the room key number, then apply those charges to the parent's credit card. Just do this before 11:00pm the night before check-out.