As PP said, it does go by nights, but that's not a problem. If doing the regular DDP, you'll have three TS to use. You could do a breakfast and dinner on your first full day and have another one to do breakfast or lunch on your last day. You could do a signature meal which would take 2 of those 3 credits. You can use your QS for a late dinner on your arrival, then for breakfast or lunch on your departure day. So many options of how to use those credits in such a short time.
Sorry, as noted there is no way to adjust the number of credits allocated. If you stay 3 nights you will have to pay for 3 nights worth of credits. But, you can adjust the way you use them to ensure they get used.
Thanks for the input, we were really on the fence, but it looks like we could save some $ if we use them all. We have a 9,6, and 2 year old and they could easily split the 2 kids meals