Decode the barcode in your SetSail pass - it should decode to <booking#> - GTY or cabin number.
On an iPhone simply put it into your wallet and will automatically decode. On Android you’ll need a barcode decoder. Same online.
What transportation to use depends on the kids’ ability to help and the final destination. If they can help and you are going to some place near a train station, the train could be an option. If not, plan for a private car. The ports of call section or your Roll Call thread may have some details.
You are comparing to the on board price incorrectly - you can stretch the 24 Voom package across two days (likely sea days) and you can skip the first day as you will have land based data service till about dinner time. Once you factor that in, the break even is in favor of using your D discounts on board vs. buying ahead (unless you get some crazy discount like 50% off).
Assuming all of you are fixed traditional (early or late) you can email rcldining@rccl.com about two weeks out to confirm the table and location - include all booking numbers.