Hi there, Tom! Welcome to
planDisney, and thanks a bunch for bringing us your question!
The tricky part here is that there isn't a fixed, universal price for upgrading from the Quick-Service Dining Plan to the
Disney Dining Plan. The cost can vary quite a bit depending on several factors that are specific to your reservation, like your travel dates, the length of your stay, your party size, and even the particular offer attached to your booking.
Because of that, there isn't a set per-adult or per-child number that I can confidently give you that would apply to your trip. The pricing is calculated individually based on your exact package, so it really needs to be looked at on a case-by-case basis.
The best way to get a precise answer is to reach out directly to the
Walt Disney World help center. A Cast Member will be able to pull up your reservation, review the details of the Free Dining offer once it's released, and tell you exactly what the price difference would be to upgrade for both adults and children in your party. They can also walk you through whether that upgrade makes sense based on how you plan to dine during your trip.
Tom, I know it's not the simplest answer, but once you have that personalized quote in front of you, it becomes much easier to decide what works best for your vacation style. I hope your planning process is just as fun as the trip itself.
Warm hugs,
Ana Paula