I find the easiest way is to do the automatic gratuities, then at the end of the cruise or whenever I want to give someone something extra I do. The crew members who work the rock wall or ice skating rink are a little higher up on the food chain and make a good deal more than the regular crew members. However, if your grandchildren receive extra special treatment, there is no harm in giving a tip. I always tip the excursion drivers/boat captains/crew members, etc. IF they provide a great excursion.
There are some that tip the first or second night, and then again at the end, but I prefer to tip above the automatic gratuities at the time of service. I have, on occasion, given an extra $20 or so on the last night to some of the crew. Usually a favorite bar tender, almost always my cabin steward and their assistant, sometimes the head waiter or maitre d'. It really all depends on the service I receive.