Hi Thomas!
You could do that, but my recommendation would be, see if there's a temporary account you can open at your bank (hopefully at no addition cost), and put the money there until you have enough to cover everyone.
Each time you purchase a ticket, you'll be charged a sales tax fee and shipping (if you decide to ship vs. doing Will Call, which I'll explain in a bit).
The shipping is generally a standard fee regardless of how many tickets you purchase, and the sales tax is based on ticket days, not how many people are coming. So it'd be to your advantage to purchase them all at once when the time comes.
As for Will Call, this is a delivery option you can choose where you pick up your ticket at any theme park booth when you get to Disney World. This can be used at no additional cost.
Hope this helps with your planning! You and your family enjoy your visit!