hey pal, welcome to the world of disney cakes!
since you have someone helping, i would suggest that the easiest option might just be to send your niece with a gift card to cover the cost of the cake? i prepaid last trip and had no problems (GF, not Boardwalk), but a gift card would ensure no hiccups.
edited to say i see you’ve already gotten things sorted (yay!). i’m leaving my comment though, just in case it helps anyone else in the future. ask your family to share a pic so we can see the creation!