With all due respect, the kids shouldn't get a say even though you're going for them. Ofcourse they will want GC. They've been before. They enjoyed it. Theme parks are great.
But I will gaurantee they'll love the cruise as much if not more. But they couldn't possibly know that without trying it.
Gratuity are interesting. I've used Scenic before and it gets added to your daily account. About $10/day/person. It's automatic but you can alter it by request. Nobody will know or care if you alter it to $0 but most are happy to pay.
Basically you give credit card detail when booking and get issued a card to use on ship. You never use cash for drinks. It's tallied up daily and settled at the end via credit card.
All cruises originating overseas do it that way. Australian based cruises can be different. You still pay the gratuity but often incorporated into you original booking. Prepaid at the same rate around $10/day.
I'm assuming for the cost that you mention and the length that it's Oz-to-NZ or maybe Singapore is cheap. Budget $20/day for you and the misus. Maybe nothing from the kids.
So you $100 is more than enough. Softdrinks are generally included. So it's only alcohol or gift shop. That wont burn the pocket.
Excursions? Depends where you're going and whats on offer. When we went cruising Canada we went by sea plane one day. Also did ballooning. Often lunch isn't included if you leave the ship. So you eatr out at your expense.
Plenty for the kids to do onboard. Plenty for adults. Most have cinema and casino. All depends.
