As KristiMC said, gratuities are $12 per person, per night. This includes infants and I think you will find that both your stateroom host/ess and dining team will bend over backwards to get you what you need for the little ones.
You should head to Guest Services sometime after you board to apply the gift cards to your account. We like to go on the first afternoon or second morning...whenever we see there is no line. It will take a few minutes to apply the cards and then you'll be all set. Hang on to the cards though. If, for any reason, you don't use all of the funds you apply to your account they will go back to one or more of your gift cards.