Hi, Marita!
Hollywood & Vine is a great place for your family to enjoy wonderful food while touring Disney's Hollywood Studios. We love buffet options, especially when we have little ones with us (they don't have to wait for the food to be served!) so this is a great choice.
At this location, you'll need to secure the reservation with a credit card. This means that if you don't show up for your reservation and fail to cancel at least 24 in advance, you'll be charged $10 per person on the reservation. I've had plans change on both of my trips to Walt Disney World thus far this year, needing to cancel at least one Table Service reservation (with credit card guarantee) per trip. On both occasions, I canceled at least the day prior to the evening reservation with no trouble, no fee whatsoever.
If you need to reduce the number of diners attending your reservation, you can modify your existing reservation to reflect your needs as well. (The opposite, though, is difficult, as the restaurant may not have seating for additional members of your party; a reduction in your party size may actually free up a table for a family who'd like to dine there at the last minute.)
I suggest that you book your ADR as soon as possible, securing the table for the entire party. (Remind them that the food is excellent, and is geared for all ages.) Should you determine that some in the group would rather not attend, you can then adjust your reservation for your smaller party size. Win-win!
Send more questions as you think of them, Marita; we love to help!