Just to add what has already been posted, my wife and I usually figure about $100 per night on the cruise to cover tips and drinks and a few onboard purchases. The costs have risen a bit so that price is probably a bit higher than that. However we rarely do shore tours. We do buy a wine package for dinners. We are average drinkers. My wife gets a soda card. I think we come close to break even on that, but the convenience is worth the few extra dollars we may spend. Of course this average bill ($700 for a 7 night cruise) is somewhat dependent on number of days at sea. We tend to spend more at sea on the ship than in port for the obvious reasons. Also if your ship goes to Labadee or Coco Cay, all costs of drinks are charged to the ship account so that is like another "at sea" day.
If you look at RCL website you can sort of discuss any shore tours you are interested in and get a feel for the cost. Tips are about $10 per day per person, higher in a suite. Also we always add extra tips for our waiters and room steward for a job well done. Watch for sales on the ship. If you buy alcohol to bring home the ship will generally guarantee they will meet any price on shore if you show them proof. We took a photo of a price in St. Thomas and the ship refunded our purchase to match the price from shore. RCL is very good in my experience for price guarantees. They also hold sales of various items throughout the sailing, so watch for those.
Hope this helps. If your kids are like mine were on our first cruise, they will love you forever for this. Its wonderfully addicting.
One last comment. If you compare the price of cruising with the price of resorts, you will see what a great deal it is. Don't be afraid to spend on this vacation. You deserve to have a great time. It does seem like they "nickel and dime" us but when you look at the costs to cruise versus the cost to run the ship, versus the cost of a resort, it is a very economical vacation. Have a great time.